Hypocalcemia and hyperphosphatemia caused by parathyroid hormone (PTH)-resistance are the only discernible abnormalities in pseudohypoparathyroidism type Ib (PHP-Ib). Because mutations in the PTH/PTH-related peptide receptor, a plausible candidate gene, had been excluded previously, we conducted a genome-wide search with four PHP-Ib kindreds and established linkage to a small telomeric region on chromosome 20q, which contains the stimulatory G protein gene. We, furthermore, showed that the genetic defect is imprinted paternally and thus is inherited in the same mode as the PTH-resistant hypocalcemia in kindreds with PHP-Ia and/or pseudo-pseudohypoparathyroidism, two related disorders caused by different stimulatory G protein mutations.