The melanocortin peptides and their receptors represent one of the most complex systems in human endocrinology. Hormonal regulation includes pigmentation, weight maintenance, adrenal function, and exocrine gland secretion via endocrine, paracrine, autocrine, and neurocrine action of melanocortin peptides at five different but homologous melanocortin receptors. Genetic relevance of the melanocortin system for human physiology was initially shown by mutations in the different melanocortin receptor genes, first described in the melanocortin-2 receptor gene in 1993 as one reason for congenital hypocortisolism. Because all ligands within the melanocortin systems are derived from one single precursor hormone, proopiomelanocortin (POMC), a genetic defect in the POMC gene could have been expected to affect all functional components of the melanocortin system. Accordingly, patients with a complete defect of the POMC gene product due to homozygous or compound heterozygous loss of function mutations were shown to be affected mainly by red hair, early-onset obesity, and congenital hypocortisolism. No further obvious clinical problems were described in these patients, suggesting that no additional function of the melanocortin system has escaped recognition. However, whether partial loss of function mutations in the POMC gene might lead to more circumscribed phenotypes, especially common obesity, remains an open question.