Prosensa Patient Group Communication: Prosensa Regains Rights to Drisapersen

Dear Patient Group Representative,

We are writing as we would like to share with you that as of January 12, 2014, Prosensa has regained

the rights to drisapersen from GSK and retained the rights to the other DMD programs.

Prosensa and GSK have issued a press release to announce this news, and I have attached the

release for your reference. Study investigators have also been notified of these changes and are

recommending that boys and their families currently participating in drisapersen clinical studies

contact their local clinical study team.

Prosensa is now in a position to decide on the best path forward for drisapersen, based on the

results of the analysis of the drisapersen studies, which is still ongoing. Prosensa will be receiving

the full drisapersen data set under the terms and conditions of the agreement, and we will work

continuously to make an assessment on these data internally and determine a path forward for

drisapersen in a timely manner. We will continue to work closely with patient groups, investigators,

academia and regulators to ensure an optimal outcome for all of our DMD programs. As soon as we

have additional clarity on the dataset and the path forward for drisapersen, we will communicate

this. The work that we do and will continue to do will hopefully bring us one step closer to finding

the right treatment options and bring these to patients.

Prosensa will be hosting a patient focused webinar at 8 AM ET/2 PM CET on Tuesday January 21,

2014 to discuss the data we have seen thus far. Details of the webinar can be found in the

“Investors & Media” Section of the Prosensa website under “Events.”

Your selfless contribution to the study of DMD has enabled us to cultivate the largest global dataset

to date for DMD, which will hopefully assist in expediting drug development for this disease. With no

disease modifying therapies currently available for patients with DMD, it is of critical importance to

work to advance possible treatment options for these courageous boys and their families. We

remain committed to working with experts in the field to accomplish this essential goal. Our primary

focus remains with these patients and their families and we will continue to work on ways to enable

them a brighter future.

We are humbled by the overwhelming support we have received from the patient community at large and express our sincerest gratitude for all of the encouragement and assistance your organization has provided to families.

Kind regards

Giles Campion, MD – Chief Medical Officer and SVP Research &DevelopmentClaire Leyten, PharmD – Manager Patient Group Relations