Knockout (KO) mice models have generated a wealth of new information on the developmental and physiopathological roles of several hormones and their receptors. In these mice, KO of a specific gene can be lethal at embryonic stages or during early adulthood. Furthermore, in conditions of non-lethality, KO mice may compensate for the repression of a particular protein expression. As a result of these two aspects, various phenotypic expressions occur in KO mice models for several peptides and their respective receptors, as well as for the enzymes involved in their processing.