C9orf72 and amyotrophic lateral sclerosis: While for the other ALS-related proteins such as SOD1 and TDP-43 a prion-like behavior is well established, data indicating an intercellular spreading mechanism for FUS and C9orf72-derived DPRs is only recently emerging (Nomura et al., 2014; Feuillette et al., 2017; Zhou et al., 2017; Morón-Oset et al., 2019).