DRD2 and obesity due to melanocortin 4 receptor deficiency: The A1 allele has been shown to be associated with low DRD2 density in human brain, compared with the A2 allele.37 It has been hypothesized that such persons who have a reduced DRD2 density have a deficit in their reward system and experience an enhanced reward when exposed to dopaminergic agent, thereby making them more prone to nicotine dependence.38,39 Therefore, the A1 allele (or low DRD2 density) has been reported to be significantly associated with increased risk of alcoholism,40,41 abuse of various drugs42-44 and obesity.45,46