The C9orf72 (C9) nucleotide repeat expansion (NRE) mutation, consisting of a repeated hexanucleotide (G4C2)n located in a classically noncoding region of the gene, is the most prevalent genetic mutation associated with the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD)1,2. The gene discussed is C9orf72; the disease is frontotemporal dementia.