C9orf72 and amyotrophic lateral sclerosis: Tremendous progress and hypothesis have greatly contributed to our understanding of the pathologic mechanism of C9orf72 hexanucleotide repeat expansions in ALS/FTD (Edbauer and Haass, 2016; Gitler and Tsuiji, 2016; Haeusler et al., 2016; Goodman et al., 2019).