DRD2 and nicotine dependence: Our top gene associated with lifetime cannabis use was NCAM1, a known candidate for nicotine dependence.45 The role of NCAM1 is to regulate pituitary growth hormone secretion as a membrane-bound glycoprotein that mediates cell–cell contact by hemophilic interactions.46NCAM1 is part of the NCAM1–TTC12–ANKK1–DRD2 (NTAD) gene cluster, which is related to neurogenesis and dopaminergic neurotransmission.