C9orf72 and amyotrophic lateral sclerosis: The possible pathological mechanisms by which the C9ORF72 repeat expansion can lead to FTD and ALS are: 1) hypermethylation of the repeat expansion and the CpG promoter region of the C9ORF72 gene leading to haploinsufficiency [5, 43], 2) retention of repeat containing intron 1 in mRNAs causing RNA foci to appear in both nucleus and cytoplasm that sequester RNA-binding proteins [11, 27] or 3) the production of dipeptide repeats (DPR) by unconventional repeat-associated non-AUG (RAN) translation of the repeat [2, 18, 31].