Furthermore, gene-to-gene interactions between the mu-opioid receptor (MOP) gene (OPRM1) and the serotonin transporter (5-HTT) or 5-HT1A (HTR1A) genes had antagonistic effects on endogenous descending pain modulation in healthy subjects and in fibromyalgia patients [12]. The gene discussed is HTR1A; the disease is fibromyalgia.