C9orf72 and amyotrophic lateral sclerosis: Beyond C9ORF72, studies have identified CAG repeat expansion in the ataxin-2 gene (ATXN2) as a genetic cause of spinocerebellar ataxia type 2 (SCA2) and ALS (Elden et al., 2010; Lee et al., 2011; Li et al., 2016; Sproviero et al., 2017).