An Extension Study to Evaluate the Long-Term Safety and Clinical Activity of mRNA-3705 in Participants Previously Enrolled in Other Clinical Studies of mRNA-3705

Participants with isolated MMA due to MUT deficiency who were previously enrolled in other clinical studies of mRNA-3705 will have the option to enroll into this extension study provided all eligibility criteria have been met. The study will include 2 periods:

1. Treatment Period and 2) Follow-up Period (up to 6 months after the last dose of study drug).

Treatment Period will continue unless one of the following occurs: mRNA-3705 receives marketing approval and reimbursement in the country of origin of the participant (following market approval and access being in place, all participants who wish to continue treatment will be offered mRNA-3705 through market access, with the intention of preventing treatment interruption. Other appropriate access program is offered as allowed by local regulation to continue treatment with mRNA-3705. Safety monitoring will be performed for all participants under treatment per market access requirements), the participant discontinues study drug, the participant is no longer receiving clinical benefit in the opinion of the Investigator, or Sponsor discontinues the development of mRNA-3705.