SOD1 and Parkinson disease: Impaired mtDNA methylation has been observed in various conditions characterized by increased oxidative stress and/or impaired one-carbon metabolism, such as blood DNA samples from ALS individuals with SOD1 gene mutations [67], the postmortem brains of AD and PD patients [47], animal models of AD and ALS [29,47,73], and blood DNA samples from patients with MCI and AD [49], aging [39,42], cancer [44,45], and metabolic disorders [74,75], among others.