C9orf72 and amyotrophic lateral sclerosis: Finally, the mutations may downregulate expression of C9orf72 protein; C9orf72 is believed to function in autophagy and/or endosomal trafficking, and these processes are known to be damaged in ALS/FTD (Balendra & Isaacs, 2018; Braems et al., 2020; Cook & Petrucelli, 2019).