The repeat expansion in the C9ORF72 gene is a major genetic factor in ALS and FTD, accounting for approximately 40% of familial ALS (FALS), 30% of familial FTD (FFTD), and about 8% of sporadic ALS (SALS) cases, particularly in predominantly Caucasian populations. This evidence concerns the gene C9orf72 and frontotemporal dementia.