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Background A subset of familial isolated primary hyperparathyroidism (FIHP) is a variant of hyperparathyroidism-jaw tumor syndrome (HPT-JT). Aim/Patients and Methods We investigated the involvement of the HRPT2, MEN1, and CASR genes in provisional 11 FIHP families and 2 HPT-JT families. Results Germline mutations of HRPT2 were found in 2 of 11 FIHP families and 1 of 2 HPT-JT families. One FIHP family with parathyroid carcinoma and atypical adenomas, and another FIHP family with cystic parathyroid adenoma had novel frameshift mutations of 518-521del and 62-66del, respectively. In a patient with HPT-JT, a de novo germline mutation of 39delC was detected. Novel somatic HRPT2 mutations of 70-73del and 95-102del were found in 2 of 5 parathyroid tumors in a family with 518-521del mutation. Biallelic inactivation of HRPT2 by a combination of germline mutation and somatic mutation was confirmed in parathyroid tumors. The finding that 2 families diagnosed with FIHP carried HRPT2 mutations suggests that they have occult HPT-JT. In the remaining 10 families, one family had a missense MEN1 mutation. No mutations of CASR were detected. Conclusion Our results confirm the need to test for HRPT2 in FIHP families, especially in those with parathyroid carcinomas, atypical adenomas, or adenomas with cystic change. more...