Studies have reported that the functional polymorphisms rs6356 in TH, rs921451 in DDC, rs4680 in COMT, rs1799836 in MAOB, and rs1611115 in DBH are associated with the activities of their respective enzymes and influence PD susceptibility and/or its phenotypes (Kurth et al., 1993; Healy et al., 2004). This evidence concerns the gene COMT and Parkinson disease.