July 20, 2026

Dear Duchenne Community:

Today, we are pleased to share that the U.S. Food and Drug Administration (FDA) has accepted for review the Biologics License Application (BLA) for zeleciment rostudirsen (z-rostudirsen, also known as DYNE-251)* for the treatment of individuals living with Duchenne muscular dystrophy (DMD) who are amenable to exon 51 skipping.

The FDA has granted the application Priority Review and assigned a Prescription Drug User Fee Act (PDUFA) target action date of January 21, 2027. As part of the standard regulatory process, when the FDA accepts a BLA, it has determined that the application is sufficiently complete and ready to undergo a thorough review. It is important to note that the acceptance of a BLA does not guarantee approval by the FDA.

Assuming approval is received, and it occurs on the anticipated timeline, Dyne continues to expect a potential U.S. launch of z-rostudirsen in the first quarter of 2027.

This milestone would not have been possible without the dedication of the Duchenne community, including individuals living with Duchenne, families, advocates, healthcare professionals, and clinical trial participants. We are deeply grateful for the partnership and support that have helped bring this potential therapy to this point.

You can read our full press release here.

About Zeleciment Rostudirsen (z-rostudirsen, also known as DYNE-251)

• Z-rostudirsen is an investigational therapy being developed for individuals with DMD who have mutations in the DMD gene that are amenable to exon 51 skipping.

• The BLA includes data from the positive Phase 1/2 DELIVER clinical trial. Based on these data, we are seeking Accelerated Approval. The FDA granted the BLA Priority Review, which is granted to medicines that, if approved, could offer significant improvements in the treatment of a serious condition. Priority Review reduces the review process from 10 months to 6 months.

• Z-rostudirsen continues to be evaluated in the long-term extension portion of the DELIVER trial and in the global confirmatory Phase 3 FORZETTO clinical trial.

• Z-rostudirsen is designed to enable the production of near-full length dystrophin in muscle and the central nervous system (CNS) to potentially provide functional improvement across a broad set of measures.

• Z-rostudirsen is administered intravenously every four weeks, as proposed in the submitted BLA.

In addition to z-rostudirsen, Dyne is advancing four development candidates—DYNE-253, DYNE-245, DYNE-244, and DYNE-255—for the potential treatment of individuals with DMD amenable to exon 53, 45, 44, and 55 skipping, respectively.

For additional information, please contact us at patientadvocacy@dyne-tx.com.

The development of z-rostudirsen has been informed by meaningful input from the Duchenne community. In partnership with Duchenne patient organizations around the world, we remain committed to transparent communication and to providing regular updates on our regulatory and clinical progress.

With gratitude,

The Dyne Therapeutics Team

* Z-rostudirsen is an investigational therapeutic and has not been approved by the FDA or any other regulatory authority for commercial use, and the safety and efficacy of z-rostudirsen have not been established.