A Study for Participants With Spinal Muscular Atrophy (SMA) Who Previously Participated in Nusinersen (ISIS 396443) Investigational Studies

Key Inclusion Criteria:

• Signed informed consent of parent or guardian and signed informed assent ofparticipant, if indicated per participant's age and institutional guidelines.

• Completion of the index study in accordance with the study protocol or as a resultof Sponsor decision (e.g., early termination of the index study) within thepreceding 16 weeks

Key Exclusion Criteria:

• Have any condition or worsening condition which in the opinion of the Investigatorwould make the participant unsuitable for enrollment, or could interfere with theparticipant participating in or completing the study

• Clinically significant abnormalities in hematology or clinical chemistry parametersor electrocardiogram (ECG), as assessed by the Site Investigator, at the Screeningvisit that would render the participant unsuitable for participation in the study

• Participant's parent or legal guardian is not willing or able to meet standard ofcare guidelines (including vaccinations and respiratory syncytial virus prophylaxisif available), nor provide nutritional and respiratory support throughout the study

• Treatment with another investigational agent, biological agent, or device within onemonth of Screening, or 5 half-lives of study agent, whichever is longer

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.