Publisher Summary Studies on parathyroid hormone receptor (PTHrP) and PTH/PTHrP receptor-ablated mice, and in transgenic animals overexpressing PTHrP in the growth plate, have predicted that human disorders caused by mutations in either of these two proteins would be associated with severe abnormalities in endochondral bone formation and in the regulation of mineral ion homeostasis. These insights from genetically manipulated animals have led to the identification of activating and inactivating PTH/ PTHrP receptor mutations in two rare genetic disorders, Jansen's and Blomstrand's disease, respectively. More recently, mutations in the PTH/PTHrP receptor gene have been found in two other diseases, namely Eiken familial skeletal dysplasia and Ollier's disease. In addition to resolving the pathogenesis of puzzling human disorders, these naturally occurring PTH/PTHrP receptor mutations have provided important new insights into the importance of this G protein-coupled receptor in mammalian development. The availability of mutant, constitutively active PTH/PTHrP receptors has furthermore provided novel tools to study bone and cartilage development independent of PTH and PTHrP.