This study will evaluate the pharmacokinetics (PK) and safety of risdiplam in participants with spinal muscular atrophy (SMA) under 20 days of age at first dose.

Inclusion Criteria:

• Male or female newborn infant aged <20 days at first dose
• Newborn infants with genetic diagnosis of 5q-autosomal recessive SMA or newborn infants identified as positive for SMA via newborn screening or via prenatal testing.
• Gestational age equal to or greater than 37 weeks
• Receiving adequate nutrition and hydration at the time of screening
• Adequately recovered from any acute illness at baseline and considered well enough to participate in the study
• Parent/caregiver is willing to consider nasogastric, nasojejunal, or gastrostomy tube placement during the study to maintain safe hydration, nutrition, and treatment delivery, if recommended by the investigator.

Exclusion Criteria:

• Presence of clinical symptoms or signs consistent with SMA Type 0
• In the opinion of the investigator, inadequate venous or capillary blood access for the study procedures
• Systolic blood pressure or diastolic blood pressure or heart rate abnormalities
• Presence of clinically relevant electrocardiogram (ECG) abnormalities
• The infant (or the person breastfeeding the infant) taking any of the following: any inhibitor of CYP3A4 taken within 2 weeks (or within 5 times the elimination half-life, whichever is longer) prior to dosing, any inducer of CYP3A4 taken within 4 weeks (or within 5 times the elimination half-life, whichever is longer prior to dosing, and/or use of any multidrug and toxin extrusion (MATE) substrates taken within 2 weeks (or within 5 times the elimination half-life, whichever is longer) prior to dosing
• Concurrent or previous administration of nusinersen or onasemnogene abeparvovec
• Clinically significant abnormalities in laboratory test

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing