C9orf72 and amyotrophic lateral sclerosis: The most common genetic cause of ALS is the hexanucleotide repeat expansions in the C9orf72 gene: in this case, pathogenesis derives from both a gain-of-function effect from the repeat DNA, sense and antisense repeat RNA, and dipeptide repeat proteins and a loss of function of the C9orf72 protein effect, thus altering a diversity of cellular pathways [157].