A pathological hexanucleotide repeat expansion (GGGGCC), in the intronic region of the C9orf72 locus—exceeding 30 repeats—is the most common genetic cause of familial autosomal dominant amyotrophic lateral sclerosis (ALS) as well as frontotemporal dementia (FTD) with ALS [116]. The gene discussed is C9orf72; the disease is frontotemporal dementia.