Results from the present study suggest that the mtDNA D-loop methylation levels in sporadic ALS patients are more similar to SOD1-mutant patients than to C9orf72-ALS patients, given that both sporadic and SOD1-related ALS patients had lower mitochondrial methylation levels compared to control subjects and both differed significantly from the methylation levels observed in C9orf72-ALS patients. Here, C9orf72 is linked to amyotrophic lateral sclerosis.