MTHFR and hemochromatosis type 1: Studies have shown that Nrf2, nitric oxide synthase (NOS), manganese superoxide dismutase (MnSOD), cytochrome P450s (CYPs), hemochromatosis (HFE) and methylenetetrahydrofolate reductase (MTHFR) participate in the development and progress of PD through pathways related to the oxidative stress, including mitochondrial dysfunction, DNA damage, nerve cell apoptosis, and neuroinflammation (23–27).