A hexanucleotide repeat expansion mutation in chromosome 9 open reading frame 72 (C9orf72) is the most common known genetic cause of amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease characterized by the progressive loss of corticospinal, brainstem and spinal motor neurons. The gene discussed is C9orf72; the disease is amyotrophic lateral sclerosis.