The primary objective of this study is to evaluate the safety and tolerability of BIIB078 in adults with C9ORF72-ALS. The secondary objective of this study is to evaluate the pharmacokinetic profile of BIIB078. As the first-in-human study, the study enrolls a small number of patients in each cohort. Every patient in a cohort is treated with the same dose or placebo. The study is designed to evaluate and confirm the safety of each dose before enrolling and exposing new patients to a higher dose in the next cohort. Therefore, proceeding from cohort to cohort, the recruitment status of "Active, not recruiting" may change. Please contact clinicaltrials@biogen.com for further information