DRD2 and Borderline personality disorder: For example, BPD patients have shown higher methylation in several genes involved in monoamine neurotransmission, including the serotonin receptor genes HTR2A and HTR3A, the dopamine D2 receptor gene DRD2, and enzymes like MAOA, MAOB, and COMT that break down monoamine neurotransmitters [20,26,27,29,34].