Key Inclusion Criteria:  For PwSMA  Genetic documentation of 5q SMA (homozygous gene deletion, mutation, or compound heterozygote).  For PwALS  ALS diagnosis by a board-certified neurologist. The minimal level of diagnostic certainty should be "possible ALS" according to El Escorial criteria.  For Healthy Participants  Age group matched with SMA and ALS participants  Key Exclusion Criteria:  For PwSMA and PwALS  Change of disease modifying treatment (DMT) in the last 1 month. Recent history of bacterial meningitis, viral encephalitis, or hydrocephalus. Addiction (alcohol or another drug abuse). Presence of an implanted shunt for the drainage of cerebrospinal fluid (CSF) or of an implanted central nervous system (CNS) catheter. Hospitalization for surgery (i.e., scoliosis surgery or other surgery), pulmonary event, or nutritional support in the previous 2 months or planned within the study duration. Known pregnancy.  NOTE: Other protocol defined inclusion/ exclusion criteria may apply.