• Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information (PHI) in accordance with national and local subject privacy regulations.

• Males and females with SMA type II or type III, aged 18 to 70 years at the time of enrollment.

• Genetic documentation of 5Q SMA homozygous gene deletion, mutation, or compound heterozygote.

• Are treatment naïve for SPINRAZA® (nusinersen).

• Have been prescribed SPINRAZA® (nusinersen) by the treating physician as part of their clinical care for SMA following the FDA approved prescribing information guidelines as follows: dose level (12 mg), dosing schedule (3 loading doses administered at 14-day intervals, and the fourth loading dose administered 30 days after the third dose and subsequent maintenance doses administered every 4 months) and safety lab monitoring (CBC, PT, INR, PTT, UA) done prior to each dose administration.

• Believed to be able to complete all study procedures, measurements and visits.

• Estimated life expectancy at least 30 months from first dosing, in the opinion of the Investigator.

• Revised upper limb module (RULM) score ≥ 4 (more than marginal upper extremity function/strength.

• Must meet either Group 1 or Group 2 criteria.

⁃ For Group 1 subjects:

• May be ambulatory or non-ambulatory (defined as being wheelchair reliant at least 75% of time and unable to walk at least 10 meters without assistance).

• RULM score of 4-34, inclusive.

⁃ For Group 2 subjects:

• Ability to walk at least 10 meters without assistance (i.e., four point walking aid).

• Be free of major orthopedic deformities that limit ambulation.

• An ambulatory subject can qualify for both Group 1 and Group 2 if the RULM score is ≤ 34.