Key Inclusion Criteria:  Participant has a body weight of ≥11.0 kilograms (kg) at the Screening Visit. Participant has a diagnosis of isolated MMA due to MUT deficiency confirmed by molecular genetic testing. Participant has a serum/plasma vitamin B12 level equal to or above the lower limit of normal (based on laboratory reference range) confirmed in the Screening Period. For those participants found to have an elevated serum/plasma vitamin B12 level, the participant may enter if, in the opinion of the Investigator, the cause of the elevation is secondary to B12 supplementation. Participant or his/her legally authorized representative is willing and able to provide informed consent and/or assent as mandated by local regulations and willing and able to comply with study-related assessments. Sexually active females of childbearing potential and sexually active males of reproductive potential agree to use a highly-effective method of contraception during the study and for 3 months after the last administration of study drug.  Key Exclusion Criteria:  Participant has a diagnosis of isolated MMA cb1A, cb1B, or cb1D enzymatic subtypes or methylmalonyl-CoA epimerase deficiency or combined MMA with homocystinuria. Participant has previously received gene therapy for the treatment of MMA. Participant has any clinically significant medical condition or medical history that, in the Investigator's opinion, could interfere with the interpretation of study results or limit the participant's participation in the study.