C9orf72 and amyotrophic lateral sclerosis: This mutation is predicted to cause ALS/FTD via three non-mutually exclusive mechanisms: (1) a loss-of-function mechanism due to reduced C9ORF72 protein expression (Liu et al., 2022b; Banerjee et al., 2023; Dane et al., 2023; Zhu et al., 2020), (2) a gain-of-function mechanism due to toxicity from repeat-containing sense (GGGGCC) and antisense (CCCCGG) RNA (McEachin et al., 2020; Parameswaran et al., 2022), and (3) toxicity from dipeptide repeat (DPR) proteins produced from these transcripts (Loveland et al., 2022; Taylor et al., 2016; Kwon et al., 2014; Wen et al., 2014).