The G4C2 hexanucleotide repeat expansion mutation (HREM), located in the first intron or promoter region of the C9ORF72 gene on chromosome 9p21 represents the most common mutation associated with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) in populations of European origin1, 2, 3. The gene discussed is C9orf72; the disease is amyotrophic lateral sclerosis.