SORCS1 and obesity due to melanocortin 4 receptor deficiency: While lack of MC4R leads to overt obesity with a simultaneous rise in fat and lean tissue mass 53, deficiency in MC3R (as well as in SORCS1/3) results in an increased fat and a concomitantly decreased lean mass, and in aberrant nutrient partitioning in the absence of classical obesity 50, 51.