C9orf72 and amyotrophic lateral sclerosis: Robust evidence of FTLD and ALS belonging to a disease spectrum came from the discovery that hexanucleotide repeat expansions (HREs) in the first intron of chromosome 9 open reading frame 72 gene (C9orf72) are the most common genetic cause of familial FTD (C9-FTD) and ALS (C9-ALS) or collectively C9-FTD/ALS [43, 172].