MCHR1 and obesity due to melanocortin 4 receptor deficiency: Chen et al. (2002) reported that Mchr1 KO mice on a C57BL/6 × 129SvJ mixed background were resistant to diet-induced obesity and had fat mass that was significantly lower in both male (4.7 ± 0.6 vs. 9.6 ± 1.2 g) and female (3.9 ± 0.2 vs. 5.8 ± 0.5 g) mice than that of the wild-type control.