In general, genes implicated in addiction can be categorized into those that influence the liability to experiment with the drug in the first place, and those that are directly involved in the biological processes underlying addiction once the individual has been exposed to the drug. As such, genes that are related to personality traits (such as impulsivity, risk-taking or response to stress) may predispose one to drug experimentation, while others may be involved in the initial, immediate subjective and physiological reaction to drug use, helping to determine whether drug use will continue and escalate. Genes that encode proteins involved in the brain reward system are also important in the development of dependence to various classes of drugs. Variability in the function of receptors, transporters and metabolic enzymes of the neurotransmitter systems listed above may modify the risk of drug dependence. For example, variability in the gene(s) encoding the dopamine D2 receptor (DRD2) and/or its adjacent ankyrin repeat and protein kinase domain-containing protein 1 (ANKK1) has been implicated in dependence to nicotine, heroin and cocaine, as well as alcoholism and abuse
