LEP and obesity due to melanocortin 4 receptor deficiency: Leptin is a 16-kDa polypeptide product of the LEP gene (also known as OB, OBS, LEPD) in humans, first isolated by Zhang et al. [43] in 1994 and mapped to chromosome 7 in 1995 [44], although an obesity causing mutation in mice was described long before [45, 46].