C9orf72 and amyotrophic lateral sclerosis: Therefore, in this study, we aimed to determine (i) the frequency of C9orf72 and ATXN2 mutations; (ii) whether NOP56 and AR are risk genes for ALS; and (iii) the potential associations between phenotypes and the size of nucleotide repeats within the C9orf72, ATXN2, NOP56, and AR genes.