SLC6A3 and nicotine dependence: Other genes shown to be associated with earlier smoking initiation and nicotine dependence include genes that code for nicotinic acetylcholine receptor (nACHr) subunits, the dopamine transporter gene (SLC6A3), the dopamine D3 and D4 receptor genes (DRD3, DRD4), and polymorphisms in a nicotine metabolizing enzyme gene CYP2A6 [21].