The SRK-015-002 protocol was amended under version 2.0 as follows: • Increased time between dosing and conducting motor function outcome measures in order to reduce patient burden and add scheduling flexibility. • Updated inclusion criteria to include prolonged contraception in female and male populations. • Added definition of Women of Childbearing Potential (WOCBP). • Added pregnancy test sensitivity to provide additional clarification and details on pregnancy test used in the study. • Updated exclusion criteria to ensure patients with prior hypersensitivity reaction to SRK-015 or excipients of SRK-015 are excluded and to ensure adequate washout of prior investigational drugs and potential muscle enhancing drugs. • Updated currently available treatments to include newly approved SMA treatment. • Updated endpoint language to clarify score utilization in evaluations. • Added language for several endpoints that were previously not included. • Updated Phase 1 study (SRK015-001) data to include results from the completedstudy. • Increased time between dosing and weight measurement in order to reduce patient burden and add scheduling flexibility. • Specified that height is collected at visits where the motor function outcome measures are conducted to ensure consistent collection of height throughout the study. • Updated PK and PD blood sample collection language in order to provide additional samples to further support PK, PD and/or ADA assay validation and to enhance the meaningfulness of the analyses. • Removed Crystatin A as a safety assessment. • Added data protection language to clarify procedures. • Various typographical and formatting corrections as well as corrections for consistency made throughout the document.

The SRK-015-002 protocol was amended as follows: - An optional extension period, with treatment for an additional 52 weeks, to observe long-term safety and efficacy effects of SRK-015 has been added - Minor clarifications throughout the protocol have been included

The primary reasons for amending the SRK-015-002 protocol are to: • Revise the dosing for patients in Cohort 3 from low-dose (2 mg/kg) to high-dose (20 mg/kg) SRK-015 after completion of the Treatment Period. This dosing change is based on the results of the prespecified 6-month safety and efficacy interim analyses. Due to the variability in timing of regulatory authority and/or Institutional Review Board (IRB)/Independent Ethics Committee (IEC) approvals, the timepoint at which patients who received the low dose (2 mg/ kg) will begin receiving the high dose (20 mg/ kg) during Extension Period A or B will vary. • Extend the Extension Period for an additional 52 weeks (as Extension Period B; Extension Visits EB1-15) to allow for collection and analysis of longer-term safety and efficacy data (104 weeks in total). To distinguish between the separate Extension Periods, the original Extension Period (Extension Visits E1-15) is now referred to as Extension Period A. • Add optional assessments to Unscheduled Visits, as follows: − Pharmacokinetic (PK)/Pharmacodynamic (PD)/antidrug antibody (ADA) sampling to provide the maximum confidence in population analyses and dosing predictions − Motor Function Outcome Measures and Quality-of-Life (QOL) questionnaires to allow for these assessments to be completed at a subsequent visit in the event that a protocol-specific visit is missed/skipped − Electrocardiograms (ECGs), as clinically necessary for safety

The primary reasons for amending the SRK-015-002 protocol are to: • Extend the Extension Period for an additional 52 weeks (as Extension Period C; Extension Visits EC1-15) to allow for collection and analysis of longer-term safety and efficacy data (224 weeks in total duration). • Revise the frequency for SST review of safety data based on the schedule recommended by the pharmacovigilance and medical teams.