For example, clear obesity and leanness phenotypes are observed in knockout mice lacking pro-opiomelanocortin (POMC; a precursor of α-melanocyte-stimulating hormone (MSH); Yaswen et al., 1999) and melanin-concentrating hormone (MCH; Shimada et al., 1998), respectively. Here, PMCH is linked to obesity due to melanocortin 4 receptor deficiency.