Therefore, we selected candidate polymorphisms of TNFSF15 rs4263839, OXTR rs2254298 and rs53576, OPRM1 rs1799971, HTR3A rs1062613, COMT rs4680, rs4818, rs6269, rs4633, and ADRAID rs1556832 as susceptibility loci either related to GI disorders or symptoms such as sleep deprivation, pain, anxiety, and depression [20,21,34,35,36,37,38,39]. The gene discussed is COMT; the disease is major depressive disorder.