A Study to Investigate the Safety and Efficacy of RO7204239 in Combination With Risdiplam (RO7034067) in Participants With Spinal Muscular Atrophy

Inclusion Criteria: * Age at screening: Part 1 Cohorts A (ambulant participants), B (ambulant participants), and D (non-ambulant participants): 5-10 years, inclusive; Part 1 Cohort C (ambulant participants): 2-4 years, inclusive; Part 2 (ambulant participants): 2-25 years, inclusive * Participants who have a confirmed genetic diagnosis of 5q-autosomal recessive SMA * Symptomatic SMA disease, as per investigator's clinical judgement * Participants who have received previous SMA disease-modifying therapies may be included provided that: Onasemnogene abeparvovec was received at least 90 days prior to screening. Participants should be tapered off steroids prior to receiving risdiplam. In addition, participants should have normal levels of liver function tests, coagulatory parameters, platelets, and troponin-I at 90 days after administration of onasemnogene abeparvovec or at least 1 month after tapering off corticosteroids, whichever comes later; Nusinersen last dose was received at least 90 days prior to screening; Risdiplam is switched to the investigational medicinal product (IMP) provided by the site Inclusion Criteria for Part 1 Cohorts A, B, and C and Part 2 only: * Participants who are ambulant, where ambulant is defined as able to walk/run unassisted (i.e., without the use of assistive devices such as canes, walking sticks, crutches, walkers, person/hand-held assistance, braces, orthoses, over the malleoli insoles or any other type of support) 10 meters in ≤ 30 seconds …