The GGGGCC (G4C2) hexanucleotide repeat expansion in the non-coding region of C9orf72 is the most common monogenic cause of inherited amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD)1,2, and also causes up to 10% of what appears to be sporadic ALS. The gene discussed is C9orf72; the disease is amyotrophic lateral sclerosis.