COMT and Parkinson disease: Additionally, the effect correlated with a polymorphism in methylenetetrahydrofolate reductase (MTHFR 677C > T; rs1801133) gene, which is commonly analysed with regard to Hcy/folate metabolism, resulting in the highest Hcy level among PD patients with both MTHFR 677TT and COMT-Val genotypes [92,97].