GPR21 and obesity due to melanocortin 4 receptor deficiency: Analysis of the epididymal fat pads of wild type C57BL/6J mice, a meaningful indicator of obesity-related diabetes, revealed an increase in GPR21 expression, which trended towards significance, in HFHS-fed mice (Fig. 1a), with a concurrent elevation in the macrophage marker F4/80 (Fig. 1b).