All participants who have received fordadistrogene movaparvovec in any Pfizer study will now be assessed for long-term safety in one combined study: C3391003

The study will evaluate the safety and dystrophin expression following gene therapy in boys with Duchenne Muscular Dystrophy (DMD). It is a single-arm, non-randomized, open-label study

The study will assess the safety and tolerability of fordadistrogene movaparvovec gene therapy. Approximately 10 participants will be enrolled in the study and receive a single IV infusion of PF-06939926; there is no placebo arm. The study includes boys who are at least 2 years old and less than 4 years old (including 3 year olds up until their 4th birthday). All boys will need to be negative for neutralizing antibodies against AAV9, as measured by the test done for the study as part of screening.

The primary analysis will occur when all participants have completed visits through Week 52 (or withdrawn from the study prior to Week 52). All participants will be followed in the study for 5 years after treatment with gene therapy.

Inclusion Criteria:

• Confirmed diagnosis of DMD by prior genetic testing.

Exclusion Criteria:

• Any of the following genetic abnormalities in the dystrophin gene: a. Any mutation (exon deletion, exon duplication, insertion, or point mutation) affecting any exon between exon 9 and exon 13, inclusive; OR b. A deletion that affects both exon 29 and exon 30; OR c. A deletion that affects any exons between 56-71, inclusive.
• Positive test performed by Pfizer for neutralizing antibodies to AAV9.
• Any prior treatment with gene therapy.
• Any treatment designed to increase dystrophin expression within 6 months prior to screening (including, but not limited to, exon-skipping and nonsense read through).
• Previous or current treatment with oral glucocorticoids or other immunosuppressive agents for the indication of DMD.
• Abnormality in specified laboratory tests, including blood counts, liver and kidney function.

Male participants age ≥2 to <4 years, at Screening (Visit 1)

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.