C9orf72 and amyotrophic lateral sclerosis: ALS-causing mutations have been so far identified in more than 25 genes, including Cu/Zn superoxide dismutase 1 (SOD1) [1], TAR DNA-binding protein-43 (TARDBP) [2, 3], fused in sarcoma (FUS) [4, 5], optineurin (OPTN) [6], chromosome 9 open reading frame 72 (C9orf72) [7, 8], and sequestosome 1 (SQSTM1) [9].