C9orf72 and amyotrophic lateral sclerosis: Whether decreased expression contributes to ALS pathogenesis is unknown since very little is known about the biological role of C9ORF72 other than its sequence similarity to the GDP/GTP exchange factor “Differentially Expressed in Normal and Neoplasia” (DENN) (Zhang et al., 2012a; Levine et al., 2013).