Using a ribonuclease A protection assay that has detected multiple mutations in the Ca(2+)-sensing receptor gene in familial hypocalciuric hypercalcemia and covers more than 90% of its coding region, we sought somatic mutations in this gene in a total of 44 human parathyroid tumors (23 adenomas, 4 carcinomas, 5 primary hyperplasias, and 12 secondary hyperplasias).