Parathyroid hormone (PTH) was the first hormone reported to have hypercalcemic action. The sequence of the mature PTH varies among species, although there is high sequence conservation between amino acids 1 and 34. PTH mRNA is detected most abundantly in the parathyroid glands of terrestrial vertebrates. Fish do not have a parathyroid gland, and the PTH mRNA in zebrafish was detected in the neuromasts of the lateral line during embryogenesis. PTH1R is expressed at high levels in bones and kidneys, where it mediates the classical effects of PTH on Ca2+ and Pi homeostasis. Furthermore, PTH is involved in the proliferation and differentiation of chondrocytes because mutations in PTH1R have been identified in Jansen-type metaphyseal chondrodysplasia and Blomstrand-type lethal chondrodysplasia. PTH2R is expressed primarily in the hypothalamus, and its involvement with the regulation of pituitary hormone release, and cardiovascular and renal hemodynamics, has been suggested.