C9orf72 and amyotrophic lateral sclerosis: Another major proposed gain-of-function mechanism for C9orf72 ALS is via the formation of pathogenic repeat RNA aggregates that sequester one or more RNA binding proteins, leading to altered RNA processing and metabolism in motor neurons (Cooper-Knock et al., 2015; Haeusler et al., 2014; Lee et al., 2013; Prudencio et al., 2015; Sareen et al., 2013).