A GGGGCC hexanucleotide repeat expansion in intron 1 of the C9orf72 gene was found to be a common cause of both ALS and frontotemporal dementia (FTD) (DeJesus‐Hernandez et al, 2011; Renton et al, 2011), accounting for approximately 40 and 25% of familial ALS and FTD, respectively, and approximately 6% of sporadic ALS (Majounie et al, 2012). This evidence concerns the gene C9orf72 and frontotemporal dementia.