Key points: The C9orf72 repeat expansion demonstrates incomplete penetrance and is the most common genetic cause of ALS in European‐ancestry populations, accounting for 1 in 10 cases, irrespective of the presence or absence of a family history of ALS or FTD.39, 43, 44, 45, 46, 47. The gene discussed is C9orf72; the disease is frontotemporal dementia.