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Using expression in Xenopus oocytes, Brown et al. cloned a bovine parathyroid Ca2+ sensing receptor (BoPCaRl). The 1,085 amino acid membrane protein is included in a class of putative seven transmembrane spanning structures that activate the phosphoinositol pathway through G proteins. Receptor activation presumably elevates intracellular Ca2+ to inhibit secretion of PTH. BoPCaRl has a big extracellular domain at the amino terminus. BoPCaRl is most similar to the metabotropic glutamate receptor. BoPCaRl mRNA is found in cells that have well known Ca2+ sensing function. Inheritance of one inactive Ca2+ sensing receptor gene causes familial hypocalciuric hypercalcemia (FHH) and autosomal dominant hypocalcemia. Mutations in the extracellular domain and in a portion of the third intracellular loop domain decrease the receptor’s sensitivity, causing FHH. Extracellular domain mutations increase the receptor’s activity at low Ca2+ concentration to cause autosomal familial hypocalcemia.