LETMD1 and obesity due to melanocortin 4 receptor deficiency: The Letmd1 KO mice are viable and grow normally (Figure 2B), and yet its BAT phenotype is more severe, resulting in defective BAT formation (Figures 3A, 3F, and 3K), impaired acute and chronic cold adaptation (Figures 2C and 4C), and increased susceptibility to diet-induced obesity (Figures 2H and 2I).