DRD2 and obesity due to melanocortin 4 receptor deficiency: Individuals with at least one copy of the drd2 Taq1A allele have reduced brain D2R availability of ~30–40% (Noble et al., 1991; Thompson et al., 1997) and an increased prevalence of obesity (Blum et al., 1996; Stice et al., 2008, 2010; Davis et al., 2009; Carpenter et al., 2013).