The largest proportion of inherited cases (40% ALS and 25% FTD) are caused by hexanucleotide G4C2 repeat expansions in the C9orf72 gene, which notably were identified from families with co‐occurring ALS and FTD (ALS/FTD) (DeJesus‐Hernandez et al, 2011; Renton et al, 2011; Majounie et al, 2012; Radford et al, 2015). This evidence concerns the gene C9orf72 and frontotemporal dementia.