However, in another study, significantly higher Hcy plasma levels were observed only in PD patients with MTHFR 677TT and low activity-determining COMT genotypes (based on the genotyping of four COMT single nucleotide polymorphisms—SNPs: rs4680, rs6269, rs4633 and rs4818) at the same time. The gene discussed is COMT; the disease is Parkinson disease.