COMT and late-onset Parkinson disease: Several SNP variations have been identified to be involved in pain perception in PD patients like rs4680 and rs6267 of catechol-O-methyltransferase gene (COMT) (Lin et al., 2017), rs6746030 of sodium voltage-gated channel alpha subunit 9 gene (SCN9A), rs324419 of fatty acid amide hydrolase gene (FAAH) (Greenbaum et al., 2012), and PINK1 (Gierthmuhlen et al., 2009).