Greetings and welcome to the Prosensa Holding's Fourth Quarter 2013 Earnings Conference Call. At this time, all participants are in a listen-only mode. A brief question-and-answer session will follow the formal presentation. (Operator Instructions) As a reminder, this conference is being recorded. I would now like to turn the conference over to your host, Celia Economides, Senior Director of Investor Relations. Thank you, Ms. Economides. You may begin.

Thank you, operator, and thank you all for joining us today. On behalf of Prosensa, I would like to welcome everyone to our earnings call for the year ended on December 31, 2013. With me today are Hans Schikan, Chief Executive Officer, Berndt Modig, Chief Financial Officer. And joining us for the Q&A are Giles Campion, Chief Medical Officer; and Luc Dochez, Chief Business Officer. Earlier today, we issued a press release containing results for the full year 2013, which is available on our website at prosensa.com. Today we also filed our full year financial statements and management discussion and analysis with the SEC. During today's call, we may make forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These include statements about our future expectations, clinical developments and regulatory timelines, the potential success of our product candidates and financial projections. Actual results may differ materially from those indicated by these statements. Forward-looking statements during this call include statements around our exon-skipping drug pipeline and our financial position. Such forward-looking information involves risks and uncertainties that could significantly affect expected results. These risks and uncertainties are discussed in the company's SEC filings, including, but not limited to, the company's annual report on Form 20-F. In addition, any forward-looking statements represent our views only as of today and should not be relied upon as representing our views as of any subsequent date. While we may elect to update these forward-looking statements at some point in the future, we specifically disclaim any obligation to do so even if our views change. With that, let me pass the call over to Hans.

Thank you, Celia, and good morning, good afternoon, everyone. As we approach the end of the first quarter of the year, we would like to provide you with an update on our important portfolio to where we stand with our unique exon-skipping compound, drisapersen, and provide an update on our financials. In the last earnings call, which took place in November of 2013, there have been a number of notable updates for the patient community and for Prosensa. On the 25th of November, Prosensa and Newcastle University in UK announced the award of a second Framework Programme 7, so called FP7, research grant from the European Commission. This new FP7 research grant totals approximately €6 million to support the development of imaging biomarkers for DMD. The project Developing imaging technologies for therapeutic interventions in rare diseases will be known as BIOIMAGE-NMD and is expected to run for three-and-a-half years. Earlier in the year, Prosensa and its consortium partners received a €6 million FP7 research grant to support the ongoing clinical study of its third novel DMD development candidate, PRO045. That project was also known as SCOPE-DMD. On the 10th of December last year, we announced the appointment of Dr. Georges Gemayel to the Prosensa Supervisory Board. Dr. Gemayel's expertise in the rare diseases will be of tremendous value in building our company and developing therapies for rare diseases with high unmet medical needs. His unique experience from his leadership at Genzyme will be invaluable as we continue to develop our pipeline of DMD candidates. On the 12th of December, Duchenne Policy Forum was organized by Parents Project Muscular Dystrophy, which brought together a number of key stakeholders, including senior members of the FDA, patient advocacy groups, clinicians, caretakers, key opinion leaders and industry leaders. During this forum, several statements were made by senior members of the FDA regarding their commitment to advance its recent landscape for DMD, where there are currently no disease modifying therapies available. It was further communicated by Dr. Janet Woodcock of the FDA that the FDA has asked for drisapersen data from GSK, which were provided to the agency in order to understand the potential biomarkers and the Natural History of the disease. The first quarter of 2014 has been foundational for the company. On 13th of January, we issued a joint press release with GSK, announcing that Prosensa regains rights to drisapersen from GSK and retains the rights to all other programs for the treatment of Duchenne muscular dystrophy. Prosensa now has the full unencumbered rights to continue the development of drisapersen as well as each of our other DMD programs, of which the full compounds are currently in chemical development. Both parties agreed that Prosensa is well suited to continue the development of all of the DMD programs. GSK has been a valuable development partner and we are grateful for the commitment made by company over the last four years to develop drisapersen as a potential disease modifying therapy for this devastating disease. Together we have been able to progress the largest global clinical program in DMD. We believe that we are in a favorable strategic position to advance our R&D pipeline and we look forward to continue these efforts within Prosensa. We continue to expand our knowledge of the Natural History of Duchenne and pursue this endeavor, we have now enrolled more than 90 patients in this global study. This study is aiming to enroll up to 250 patients across the US, Europe and Latin America and can be very helpful for comparative purposes for the product candidates, which we are developing in less-problem patient populations with DMD. In January of this year, we reported initial findings from further clinical data analyses of drisapersen for the treatment of DMD. The clinical program for drisapersen includes three placebo-controlled studies and two long-term open-label studies with a total of over 300 patients. In the longest term, those have been treated over four years with efficacy data reported up to 177 weeks. The data we presented in January focuses on values, integrated analysis across the overall drisapersen program as well as in six sets of the new boys mainly those who were 17 years or younger and those over 17 year olds. And the essential outcome is measured by six-minute walk test. In the post-hoc analysis of both Phase II studies, DEMAND II and DEMAND V are statistically significant and clinically meaningful treatment benefits of 31 meters with p value of 0.002 on the six-minute walk test was seeing between placebo and continuous, 6 mgs per kg once we moved to drisapersen treatment versus placebo at 24 weeks. In age grade analysis of DEMAND II and DEMAND III, although seven years are younger, a 24 meter treatment difference was seen for those on 6 mgs per kg per week of drisapersen versus placebo after 38 weeks of treatment. The preliminary analysis of the 96 weeks data from DMD114349 or DEMAND IV, from those participating in the Phase II, DEMAND III shows a 39 meter difference between those on continual treatment, 52 patients, and those who have been on placebo for 48 weeks followed by active drug, 31 patients. Those who previously participated in the DEMAND II study showed a 52 meter difference of 96 weeks and 30 and 17 patients respectively. These analyses are based on a total evident of 13 patients who had reached the 48 weeks in this extension study. These overall results suggest that earlier treatments in less progressed DMD boys illustrated by a subset of boys of seven years or younger is associated with a treatment difference and as for more progressed over boys along the relation of treatment suggested in the rate of disease progression as measured by the six-minute walk distance. These safety findings are consistent with previous observations and can be injection site reactions, proteinuria and moderate to severe thrombocytopenia. Based on these data, we are actively in contact with the clinical experts and key opinion leaders in the DMD field to discuss the totality of the data and the analyses conducted to date and to obtain their feedback. We've also been in contact with investigators and patient advocacy groups to discuss the possibility of re-dosing patients with drisapersen. The results encouraged us to engage with regulators both in US and Europe to obtain their feedback and define the part for drisapersen. The IND was transferred to Prosensa on the 18th of February. As you may recall, drisapersen received breakthrough therapy designation by the FDA in June 2013. Breakthrough therapy designation has all the features of a fast track designation, including actions to expedite development and review. Breakthrough therapy designation also enables intensive guidance from the agency during drug development and provides access to organizational commitment involving senior managers. On the 12th of January effective date of Prosensa reclaiming all rights to drisapersen, we and GSK entered into a 120-day transition periods during which joint transition team will transfer to Prosensa along with relevant data, know how and the product related to drisapersen. This is an integral process involving a number of disciplines of a perfect company like clinical, statistical and regulatory, pharmacokinetics, quality assurance, resource and developments and others. Prior to these announcements, we were closely involved on the ongoing analyses related to drisapersen. But as yet, we do not have full access to all individual patient data generated by GSK. It's a complex and large data set and must undergo a rigorous quality control process before the transfer can be complete. Upon the transfer of the R&D, which appeared on the 18th February, we communicated to patient advocacy groups of investigators regarding some immediate next steps for drisapersen clinical program. We are actively working on the re-dosing plans for boys to have them involved in all phase either on the immune treatment protocol or try extended access program, which is ultimate drugs dependence. These plans will likely take a staged approach with an aim at re-dosing an initial group of boys in the third quarter of this year. It's our plan to communicate further on these plans as they are established over the next few months. Taking initial steps is a complex path to providing a renewed access to drisapersen for patients. We understand that it is an incredibly difficult science for patients as they lay our next steps. There are several criteria for customer assessment in this process, including feedback from patients and investigators regarding the willingness and desire to go back from the drisapersen. Based on the inputs we've received thus far from both parents and investigators, we foresee that a majority of boys is willing to recommence treatment with drisapersen. Other criteria includes regeneration of the appropriate program to provide access to drug and obtaining the product approved to do so, sufficient chemical supplies to meet the needs of boys during re-dose both in the short and long term, prospect of the regulatory path forward for drisapersen. We will provide updates to the community as we complete these steps. We were invited by the umbrella organization of patient groups around the world, United Parent Projects Muscular Dystrophy through a webinar aiming to update patients groups and their members globally. This webinar is planned for Tuesday, the 25th of March. Our aim is to be as transparent as possible and we do hope that patient community will understand that recommencing dosing in boys will unfortunately take some time to the extent it can happen. We know it's not an easy task to take new steps, but we appreciate the community's patience and support as we complete this transition. Lastly, drisapersen, Dr. Craig McDonald from the University of California, Sacramento, yesterday presented the 48-week data from the DMD114876 or DEMAND V study. Recall that the treatment pace for this randomized placebo-controlled exploratory study was 24 weeks with a 24-week treatment follow-up period. The trial was conducted in 13 centers in United States. The results from the 24-week treatment phase were presented on September 24, 2014. These results indicated a clinically meaningful treatment of 27.1 meters with a p value of 0.069 in the primary endpoint, which was a six-minute walk distance at 24 meters. For a dose 6 milligram per kilogram treatment arm versus placebo, a p value of 0.05 or less is generally considered to be statistically significant. The drisapersen 6 mgs/kg/week treatment group showed a minimum increase dose baseline of 16.1 meters, while the placebo group showed a decline of 11 meters, leading to the difference of 27.1 meters. For percent-predicted six-minute walk distance, the treatment benefit was 5.2% at week 24 with a p value of 0.051. Percent-predicted aims to correct for age and height in order to distinguish normal growth of development from treatment effects. In the poster presented at the Muscular Dystrophy Association Clinical Meeting in Chicago yesterday, it was shown that the treatment benefit in the 6 mgs/kg, the treatment group was maintained at 48 weeks with minimum difference of 27.9 meters versus placebo with a p value of 0.177. The drisapersen 6 mgs/kg treatment arm showed an overall minimum increase from baseline of 14.7 meters, whereas the placebo arm showed a decline of 13.2 meters. For percent-predicted six-minute walk distance, the treatment difference was 4.8% with a p value of 0.154. The results of this study supports the use of drisapersen at a dose of 6 mgs/kg once-weekly in the treatment of boys eligible for exon 51 skipping. The maintenance of the treatment difference in the 24-week post-treatment phase is encouraging, and it's consistent with the possible slowing of disease progression in this younger less-severe patient population. PRO044, our next most advanced product candidate, addresses separate set of populatin of DMD patients. PRO044 has completed the Phase I/II study in Europe and results were presented in October 2013. We are currently evaluating steps forward in the PRO044 clinical program, also in view of the ongoing analysis of the drisapersen results. It's important for us to apply the knowledge we're getting from the drisapersen clinical program to our follow-on compounds. We have four additional earlier-stage compounds that address other distinct population of DMD patients. Of these, PRO044 ended clinical trials in the first quarter of 2013 and PRO053 ended clinical trials in mid-2013. We expect data from both of these programs in the second half of 2014. PRO052 and PRO055 are in advanced preclinical developments. We've also commenced a research program, PROSPECT, which includes a new and innovative application of our exon-skipping technology platform. This approach applies multiple exon-skipping. Our initial efforts in PROSPECT are focused on the exon 10 to 30 region in the dystrophin gene. As an example, 10 to 30 multiple skip could be applicable to 13% of all DMD patients. Proof-of-concept has been obtained in multiple patient muscle cell cultures and in-vivo studies are currently ongoing. I'd now like to turn the call over to our Chief Financial Officer, Berndt Modig, who will discuss financial highlights from the quarter. Berndt?

Thank you, Hans. The consolidated financial statements of Prosensa have been prepared in accordance with IFRS as issued by the International Accounting Standards Board, IASB. And the consolidated financial statements are presented in euros, which is the company's functional and presentation currency. All amounts mentioned are in euros unless otherwise specified. Prosensa's cash and cash equivalent as of December 31, 2013, were €82.2 million or approximately $112 million at the balance sheet date compared to €40.7 million as of December 31, 2012, and €86.7 million as of the quarter ended September 30, 2013. The increase in cash and cash equivalent in 2013 was due to €64 million in proceeds before issuance call from the issuance of 6.9 million shares of common stock upon the IPO on July 3, 2013. The company's cash consumption, excluding the cash flows from financing for the 12-month ended December 31, 2013 of €22.4 million, including the cash consumption related to the IPO. Revenue for the 12-month ended December 31, 2013, was €8.9 million compared with €7.9 million in 2012 due to increased collaboration revenue of €1.2 million offset by a slightly lower license income of €0.1 million. Effective January 12, 2014, Prosensa and GSK mutually terminated the collaboration agreement. The remaining license revenue balance of €40.5 million deferred as of December 31, 2013, will be fully released into income in the first quarter ended March 21, 2014. Research and development expense was €18.5 million for the 12-month ended December 31, 2013, compared to €14.4 million for the comparable period in 2012. While we incurred expenses for the preclinical safety studies for PRO045 and PRO053 in the 12-months ended December 31, 2012, our research and development expenses in 2013 mainly related to the ongoing Phase I/II study of PRO045 and PRO053. In the 12 months ended December 31, 2013, we also incurred expenses for the preclinical safety studies of PRO052, the Natural History and PROSPECT programs. General and administrative expense was €7.7 million for the 12 months ended December 31, 2013, compared to €4 million in 2012. The increase is primarily due to higher share-based compensation expense in 2013 of €1.1 million compared to €180,000 in 2012 and costs associated with operating as a public company. Net loss for the full year 2013 was €16.6 million or €0.51 per share compared to €9.9 million or €0.37 per share for the full year 2012. Effective January 12, 2014, we and GSL mutually terminated our collaboration agreement. As a result of the termination agreement, we have no future financial obligations towards GSK and we will not receive any further milestone payment from GSK. As part of the transaction, we will also take over a substantial amount of inventory that GSK has prepared in anticipation of the filing of drisapersen. We are evaluating the portion of this inventory if any can be used by Prosensa. And to the extent we can, it will mitigate CMC cost going forward. For 2014, the financial results of the company will be influenced by the outcome of our current interactions with regulators and that now represents potential path forward for drisapersen. Once we have more clarity on this, we will be in a position to provide updated financial guidance on 2014. With that, I'd like to turn it back over to Hans for some closing remarks. Hans?

Yeah, thank you, Berndt, and thanks to everyone for joining the call today. We remain committed to enabling a brighter future for DMD patients and to being a pioneer in the DMD space. We are encouraged by the totality of the drisapersen data. We intend to start re-dosing in initially the boys in the third quarter of this year and we will continue to work very closely with key patient groups and experts in the field to further develop our innovative portfolio. We will be engaging with regulators both in the EU and the US to obtain feedback on the potential regulatory path forward to drisapersen. We look forward to updating you as we know more. I would now like to turn the call back to the operator who will open the line for questions.

(Operator Instructions) Our first question is from the line of Paul Matteis with Leerink.

I have a couple here. First of all, I'm just wondering have you calendared a meeting with the FDA yet for the second quarter, and what does the agency have with regards to drisapersen demand predated the GSK give them everything? Do they have more than you guys right now? And then I have a follow-up. Thanks very much.

Well, first one regarding the question about planned meeting with the agency, no, there is not a planned meeting yet with the agency. But upon receiving the IND in our name, which took place in the 18th of February, we did talk reach out to the agency immediately to have a calendar meeting in our agendas. So that is the latest update I can give there. With regard to the data received by the FDA, when the FDA requested data from GSK already last year, GSK has been submitting various data to the FDA. That is the majority of the data and the FDA has been looking at this data, as also mentioned in the prepared part of this telephone conference, to look at Natural History and biomarkers, but also of course to get a good look at the drisapersen data as such. Have yet no feedback has been received from the FDA on that analysis. So that's also why we hope to have that meeting hopefully as soon as possible to getting their feedback and to discuss potential path forward.

And then I'm wondering too on the DEMAND V data, how the percent-predicted analyses were calculated. I mean understanding the concept of percent-predicted and how it controls the other variables like age and height. How did you develop the algorithm here to put these data in perspective, given that Natural History studies in Duchenne have shown kind of a variability in six-minute walk test trajectory? I'm just wondering how that analyses was developed and how you've optimized it and if anyone else has used this similar way of cutting six-minute walk test data before.

We have Giles Campion on the line as well, our Chief Medical Officer. So I'll hand over to him in a second to go in more details of how such a calculation is being done. But as mentioned, what it tries to do is correct for a number of elements, which are valuable, especially in younger boys affected by certain diseases, like Duchenne muscular dystrophy. You can imagine a younger boy who is still growing in age and in height that as a result of that growing pattern, he automatically will have a certain improvement in six-minute walk distance. And that's what the percent-predicted tries to correct for. What Giles (inaudible) an alternative look at very, very big numbers, but percent-predicted twice to correct for the elements which are variable and which are known already. And already back in 2012, Craig McDonald has presented on that publication, and he is already the primary author of this poster as well, with Giles being the last author in that poster, I'll gladly hand over to him to give some more comments on the difference between percent-predicted and actual six-minute walk distance. And by the way, Giles is currently in Chicago at the Muscular Dystrophy Association Clinical Conference. Giles?

Just to add on some further color, the percent-predicted is based on the use of a normative database and applying an algorithm called the Geiger equation. All the details of that are in the publication that, as Hans mentioned, authored by Craig McDonald. And as Hans said, it does correct for the growth of individuals. The six-minute walk depends on cadence, which is the number of steps per minute, but also stride lengths, which obviously is impacted by height. So it's a bit like the respiratory function test in children that you report those usually as percent-predicted to correct for the maturation effect and provide a baseline attempt, which you can normalize all measurements taken.

(inaudible) which makes a difference with the delay. One more point on this one. Yes, so we have applied percent-predicted as well in the other studies. And in the DEMAND II study, so that was first placebo-controlled studies, where results were presented in Cold Spring Harbor in April 2013. And there we saw both at week 24 and at week 48 as well a clinically significant percent-predicted, because that also was a younger group of boys in that study.

No, great, that's awesome. And it sounds like an interesting way to put these analyses in perspective and it's helpful. I just have one more. I'm wondering just for the pipeline for 045 and 053, when could we see some more data and what could we see? Will we just have dystrophin data like with 044 or could we see some six-minute walk test data as well?

The initial data, which will be most likely in the second half of this year will focus on dystrophin data. But Giles, maybe you want to say a few more things about the design of the studies of 044 and 053 and what we may be able to expect there.

These studies are difference from child design that was presented when we presented the 044 results in the late half, a 12-week treatment period. So although there are three patients per cohort certainly in the 045, so we may well see some changes in the six-minute walk. But given the small numbers and the relatively short duration of treatment, we won't be using the six-minute walk as design factor. It'll still be based on the dystrophin.

Our next question comes from the line of Chris Marai of Wedbush.

I was wondering maybe with respect to how you guys want to go about further developing drisa. Are you looking at potential for extended access with re-dosing later this year?

Yes, the goal is to go back to re-dosing eventually the patients. With our number of hurdles, we have to overcome first, and we expect that if those hurdles (inaudible) patients can be treated in the fourth quarter of this year already, but it will most likely be a staged approach. And in principle, there are two possibilities there to resupply drisapersen and to create access for patients. One would be under a treatment protocol either under the existing, but amended existing protocol with GSK amended to present their protocol or a new protocol of Prosensa or a treatment protocol in place or it makes less sense in some jurisdictions to then do that via an extended access program. So those are in principally the two possibilities, which one could follow. And we are actively working on those as we speak. But as I also mentioned, the key question initially was do patients want to go back on treatment with drisapersen. So we have reached out to investigators and they have reached out again to parents of the boys in the studies. Based on the dosing responses, we have reached out to majority of patients that would like to go back on treatment. On top of that, we have many individual letters from parents on a weekly basis from a group of parents who have boys in various studies, which sometimes is very compelling the arguments that we should start re-treatment very, very soon. But again, it's a very complex process. And one important element there of course is the supplies. GSK has transferred or will transfer to us drug supplies of both their product. We have to make sure that we can use some of those vials, because in certain sites, that probably will have to be destroyed. So it's quite logistical process as well there. And then of course, a very important element is to have also the regulatory path forward there. So based on all these hurdles and conditions, you could say we (inaudible) boys can be treated already in the third quarter of this year. That's the goal.

And then with respect to the extended access, you're looking at compassionate use type programs in the US and in the EU, would that be correct if you have drug supply?

Yeah, that's correct, Chris. So extended access programs will be a possibility then. And of course, it may be a possibility that in certain jurisdiction that maybe even could be done under reimbursement. So that's normally number one priority here is really to try to help patients as broadly as possible and it unfortunately takes time to get it organized.

With respect to McDonald's comments regarding a poster you could find it here in MDA, they're very encouraging. However obviously it's a limited study and it had certain goals. I'm wondering have folks such as Dr. McDonald and other experts, have you assured more data with them than often presented at the main type or the other studies?

Yeah, well, we have reached out from the moment that we global rights back on drisapersen, which was indeed when we had even tried to generate already in that first week, we have been reaching out to key opinion leaders and clinical experts in DMD. That process is still ongoing. And based on the feedback we have received from quite a number of experts in the field and our own expectation, we feel encouraged. But again, the most important element in here will be would regulators will (inaudible). And that's why it's so important that we will be able to discuss a data and our view, but also their view of the feedback they have on the data with regarding to the agencies in Europe and United States. But yes, we have reached out to quite a number of key opinion leaders, including (inaudible), and that feedback has been encouraging.

With regarding to path forward on the follow-on compounds, 045 and 053, are you looking at pivotal studies to be sort of the next studies? Is it too early to tell? And are you going to be discussing any of that with regulators as you discuss path forward for drisa?

Well, the hope has always been that follow-on programs could go to a far more expedited development program that runs on the first or maybe first two products will be approved. That's always the assumption also based on the discussions we have with regulators over the last couple of years. Now of course with the results of the Phase III trial of drisapersen, we try to understand as much as possible if we can learn from that data set and to what extent it may be applicable for the other compounds as well. But we are looking at possibilities to expedite that development. And that's also one of the reasons why we have over 90 patients recruited in Natural History, so it hopefully at a certain moment in time, it will be possible to compare treated boys with boys based on Natural History results with a lot of them in the placebo arms and studies. It's a bit immature to come to conclusion now already, because drisapersen is our number one priority for our discussions with regulators. We hope that indeed the follow-on programs can go to more expedited development programs based on the knowledge from drisapersen, but also based on the Natural History results, which we are generating as we speak. Maybe Giles would like to say something more about it as well. Giles, you at the MDA conference. Any additional input from your side on this one?

No, I don't have anything to add to that, Hans.

Our next question is from the line of Kumar Raja of Citigroup.

So once you have idea on how much drisapersen you'll need for re-dosing, how long will it take for you guys to manufacture new batches? And also how you guys had any interactions with the EMA and how is it different from what's going on with FDA?

Now first with regard to your question about the supplies, how long will it take as to manufacture new batches, that is indeed a very important element in any discussions round, because we want to prevent the situation that re-dosing would start at a certain moment in time and then we would have to go back to the parent and patients saying that we did do some roll planning and now we have to start dosing again. So that's a key question in there. Now the contract manufacturing organizations, which we are working for the overall DMD program, then we signed a deal with GSK back in 2009, GSK took over the contract we had with one of our key contract manufacturing organization. So we have already contracted with that company. And now it is important that those contracts will come to Prosensa. Then we will be in a better position to judge exactly how long the lead time will be. But normally, you should talk about low lead times based on our experience with the other compounds as well, although this situation, of course, you talk about larger scale manufacturing, larger batches that we are used to in the small programs where we are currently engaged like 044, 045 and 053 and so on. But it's an important element and we hope to be able to give more update there as we continue in those discussions also with these contract manufacturing organizations. At the same time, we have received substantial amounts from GSK or we will receive, I should say, substantial amounts from GSK in terms of drug substance and drug vials, so that should allow us to kick off re-dosing and do that for quite some time. So I don't think it will be a major hurdle. The lead time for contract manufacturing organization will have to be checked and we want to have these things in place before making any decisions or at least starting on re-dosing. And that's an important element here. Then with regarding the second question about the European Medicines Agency, we did have a so called R&D pipeline review meeting. We have the business pipeline meeting on the 26th of February, which is normally a meeting where you look at the total pipeline at EMA up to date as to what is happening in the pipeline, what can we expect in terms of products at a certain amount to come to the agency. And of course, you can imagine that in that meeting, also discuss to how to extend the drisapersen. So that is indeed what I can say about that meeting. And as we progress in these interactions with regulators in both the Europe and US, we hope to be able to say more about it in next few weeks and months.

Our next question is from the line of Debjit Chattopadhyay of Emerging Growth.

Just trying to understand the background, when you look at the drisapersen data set on a preclinical level and then you compare it with the other exons, the 53 and the 45, could you talk about the exon-skipping efficiencies between drisapersen and the other exons?

Well, the efficiency, Debjit, of 51 is based on the results of preclinical studies that we had done quite some years ago. It's generally lower than for 53 and 45 and even PRO044. But I don't think we have done direct comparison and run experiment. Maybe Giles can give some more color on that as well. But generally speaking of the efficiency in terms of skipping efficiency, 44 compound is higher than for 51. And, Giles, maybe you could say a few things about that as well.

I mean as you could imagine, there has been quite a lot of work done internally to try and optimize our design of antisense molecules. And primarily due to the collaboration we have with GSK, we've put in place a procedure whereby we started with about 60 or so molecules and then for process of elimination, selected the best candidates. And of course, within that was exon-skipping efficiency. So these molecules have been optimized to go through that process with 51.

So based on the higher exon-skipping efficiencies, do you think you can do it at higher levels and the toxicity profile of drisapersen versus the other exons?

Well, at Prosensa, it works. But at the moment, we're now in dose escalation studies. And we will be very interested to see what the results are, because of course higher efficiencies and hopefully an improved safety profile that we have used to optimize our candidates should lead to the ability to dose higher or at least to improve the therapeutic index. But that is the work we are doing at the moment through the dose escalation studies.

And on the clinic from the drisapersen clinical perspective going forward, do you think you could have two different trials, one in the younger boys, which is potentially placebo controlled and a 48-week duration, and a second trial in the older boys which is basically a single-arm trial, but a 96-week duration based on the data set that you so far in drisapersen?

Well, I think those are ongoing at the moment. And we'll obviously be informed to a large extent by the discussions we have with regulators. So I think both the options you mentioned are very reasonable. I think certainly the data that we have to date suggests that in the younger boys, we've seen a consistent effect with shorter-term treatment, and I think that was part of the relevance of the 876 results, which were presented yesterday, as they really support the European study that was done in a similar patient population. And I think the interesting thing about that study, particularly with the post-treatment phase, shows that even with six months of drug, the clinical benefit was maintained. So I think it does give us options for the design of confirmatory studies.

And maybe, Debjit, to add from my side to the answer, of course, the interaction with the regulators will be of key importance in defining that path forward, but also to discuss the need for any additional studies. And they could take various shapes. I mean of course, in the most ideal situation, it would be the possibility that we could file in this data. And by the way, I'm not giving any guidance here on whether we think that is possible, but we want to have those interactions first, especially also with the FDA, which is having access to a lot of data on drisapersen. But one possibility may be to filing data potentially with the post-approval commitment, it could also be that a confirmatory study would be needed for regulators to get their insight. We are encouraged by totality of the drisapersen data. Actually, I have mentioned all the time and we also need and intend to start re-dosing patients and we would like to have those interactions with the regulators to discuss the overall totality on the contracts of the three placebo-controlled studies and two open-label studies for drisapersen.

You talked about the Geiger equation at the beginning of the call. Is the Geiger equation equally valid in the younger boys as in older boys, or is there a six-minute walk distance cut-off beyond which it's not applicable in the R-square falls off?

Giles mentioned Geiger, so I'm going to pass that question to Giles. Giles, go ahead.

Yes. I mean the Geiger population is actually an Austrian population. And if you look at the (inaudible). It does look at slightly older boys. And I know that Natalie Goemans has had a look to another normative database. So it may be that there are subtle differences between what equation used to try and correct for height and age. But I think the general applicability is accepted.

At this time, I'll turn the floor back to Hans for his closing remarks.

Well, thank you very much, everybody, for joining this call, and we appreciate your interest in Prosensa. And as mentioned, we are encouraged by the data, are continuing our interactions with patient groups, with key opinion leaders and with regulators, and we hope to have Prosensa progressing in next couple of weeks or months. Thank you very much.

This concludes today's teleconference. You may disconnect your lines at this time. Thank you for your participation.