Since ALS patients harboring C9orf72 repeat expansions often present with cognitive/behavioral impairment [99], and C9orf72-targeted antisense oligonucleotides are currently under investigation (NCT03626012), genotyping to determine the length of the C9orf72 repeat expansion could be an important biomarker for ALS diagnosis and prognosis, and an inclusion criterion for some clinical trials. Here, C9orf72 is linked to amyotrophic lateral sclerosis.