ATRN and obesity due to melanocortin 4 receptor deficiency: Mgrn1- and Atrn-null mutant mice share strikingly similar phenotypes, including a dark coat color due to the loss of a subapical band of pheomelanin on individual hairs and an elevated basal metabolic rate that confers resistance to diet-induced obesity (Blanchard et al., 1995; Dinulescu et al., 1998; Gunn et al., 2001; He et al., 2001; Miller et al., 1997; Nagle et al., 1999; Overton and Leibel, 2011; Phan et al., 2006).