Zucker fatty (fa/fa, ZF) and spontaneously hypertensive obese Koletsky (fak/fak, SHROB) rats are animal models of obesity, and both strains have mutant leptin receptor in the fatty allele (Lepr(fa) and Lepr(fak) respectively), resulting in leptin signaling deficiency (Chua et al., 1996; Phillips et al., 1996; Takaya et al., 1996; Wu-Peng et al., 1997). The gene discussed is LEPR; the disease is obesity due to melanocortin 4 receptor deficiency.