C9orf72 and amyotrophic lateral sclerosis: The ALS group comprises 14 patients with pathogenic variants in the SOD1 gene, 13 patients with a pathogenic C9orf72 repeat expansion, and 36 sporadic cases negative for the presence of pathogenic alterations in the major ALS genes, namely SOD1, C9orf72, FUS, and TARDBP. Bisulfite pyrosequencing was used to quantify DNA methylation in the mitochondrial D-loop region, and quantitative PCR (qPCR) was used to evaluate the mtDNA copy number.