C9orf72 and amyotrophic lateral sclerosis: ,2 One of the main genetic causes of both ALS and FTLD is hexanucleotide repeat (GGGGCC (G4C2)) expansion (HRE) in the first intron of the C9ORF72 gene,3,4 which results in disease phenotypes via both gains of toxicity of RNA repeats and dipeptides and loss of function of the C9ORF72 protein.5