Expanded hexanucleotide GGGGCC repeats in the first intron of C9orf72 gene are the main genetic cause of the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) and are the most common cause of familial ALS and FTD within Europe and North America3–6. The gene discussed is C9orf72; the disease is frontotemporal dementia.