Inclusion Criteria:

   - Confirmed diagnosis of 5q-autosomal recessive SMA

   - Negative blood pregnancy test at screening and agreement to comply with measures to
   prevent pregnancy and restrictions on sperm donation

   - For Part 1: Type 2 or 3 SMA ambulant or non-ambulant

   - For Part 2: 1) Type 2 or 3 SMA non-ambulant; 2) RULM entry item A greater than or
   equal to 2; 3) ability to sit independently as assessed by item 9 of the MFM

Exclusion Criteria:

   - Concomitant or previous participation in any investigational drug or device study
   within 90 days prior to screening, or 5 half-lives of the drug, whichever is longer

   - Concomitant or previous administration of a SMN2-targeting antisense oligonucleotide,
   SMN2 splicing modifier or gene therapy either in a clinical study or as part of
   medical care

   - Any history of cell therapy

   - Hospitalization for a pulmonary event within the last 2 months or planned at time of
   screening

   - Surgery for scoliosis or hip fixation in the one year preceding screening or planned
   within the next 18 months

   - Unstable gastrointestinal, renal, hepatic, endocrine, or cardiovascular system
   diseases as considered to be clinically significant by the Investigator

   - Presence of clinically significant electrocardiogram abnormalities before study drug
   administration from average of triplicate measurement or cardiovascular disease
   indicating a safety risk for participants as determined by the Investigator

   - Any major illness within one month before the screening examination or any febrile
   illness within one week prior to screening and up to first dose administration

   - Recently initiated treatment (within less than [<] 6 months prior to randomization)
   with oral salbutamol or another beta 2-adrenergic agonist taken orally

   - Any prior use of chloroquine, hydroxychloroquine, retigabin, vigabatrin or
   thioridazine, is not allowed

   - Ascertained or presumptive hypersensitivity (e.g., anaphylactic reaction) to Risdiplam
   or to the constituents of its formulation

   - Recent history (less than one year) of ophthalmological diseases

   - Participants requiring invasive ventilation or tracheostomy