Adipocytes are characterized by secretion of many proteins such as adipocyte-derived vascular endothelial growth factor-A (VEGF-A) whose level is increased in obesity [34]; adiponectin related to insulin resistance [35]; preadipocyte factor 1 (Pref-1) that prevents fibroblasts from differentiation (adipose tissue generation) [36]; adipose-tissue secretory factor (ADSF/resistin) involved in low-density lipoprotein (LDL) level regulation [37]; and plasminogen activator inhibitor-1 (PAI-1/serpin E1) that plays a major role in insulin resistance [38]. Here, RETN is linked to obesity due to melanocortin 4 receptor deficiency.