HNMT and Parkinson disease: The single nucleotide polymorphism (SNP) rs11558538, located in the exon 4 C314T of the HNMT gene (chromosome 2q22.1, MIM 605283, gene identity 3176), which causes the amino acid substitution Thr105Ile (related to decreased enzyme activity), has been the matter of several case–control association studies trying to establish its association with the risk of developing PD.