The primary objectives of the study are to estimate and rank-order the longitudinal standardized mean changes over 6 months and over 12 months, for a set of outcome measures administered to participants with amyotrophic lateral sclerosis (ALS), in order to identify measures that are more sensitive to disease progression than Amyotrophic Lateral Sclerosis Functional Rating Scale-Revised (ALSFRS-R). The secondary objectives of this study are: To evaluate the test-retest reproducibility of each outcome measure; To determine correlations between 6 and 12-month changes in all exploratory measures with 18 and 24-month changes in ALSFRS-R and survival; To assess correlations between/among the various measures; To obtain biological samples in order to identify molecular correlates to the clinical measures and to further characterize previously identified and novel molecular biomarkers of disease progression for incorporation into future clinical studies.

Key Inclusion Criteria:

• A diagnosis of sporadic or familial ALS
• ALS onset within ≤5 years
• Must be 16 to 85 years of age, inclusive, for sites in the United States and 18 to 85 years of age, inclusive, for all sites outside of the United States

Key Exclusion Criteria:

• History of or positive test result at Screening for human immunodeficiency virus (HIV)
• History of or positive test result at Screening for hepatitis C virus (HCV) antibody or hepatitis B virus (HBV)
• Possibility of neuromuscular weakness other than ALS
• Unspecified reasons that, in the opinion of the site Investigator, make the subject unsuitable for enrollment or unlikely to be able to complete, at a minimum, the Month 6 Visit

NOTE: Other protocol defined Inclusion/ Exclusion criteria may apply

Participants suffering from ALS are recruited by participating physicians in a standard clinical practice setting.