We screened three unrelated Danish families with familial hypocalciuric hypercalcemia (FHH) for mutations in the Ca2+-sensing receptor (CASR) gene by polymerase chain reaction amplification and DNA sequencing of exons 2-7, which include the entire coding region of the gene. In one family the affected individuals have a T-->C mutation that changes the normal arginine at codon 220 to a tryptophan. In the other two FHH families, affected individuals have the same A-->G mutation, leading to conversion of the normal glycine at codon 552 to an arginine. These results confirm that FHH can be caused by non-conservative missense mutations in the CASR gene leading to abnormal calcium homeostasis. Both mutations are located in the amino-terminal extracellular domain of the receptor, which contains the binding site for extracellular Ca2+, the CASR's principal physiological agonist.