Less than 5% of patients with PHPT occur as syndromic forms of PHPT, belonging to complex familial disorders comprising multiple endocrine neoplasm syndrome types 1 to 4 (MEN1–4), hyperparathyroidism-jaw tumour syndrome and FHH [30], which may be caused by heterozygous germline mutations of the MEN1 (chromosomal position: 11q13), RET (10q11.2), CDKN1B (12q13) and HRPT2 (CDC73) (1q31.2) or CASR (3q21.1) gene. This evidence concerns the gene MEN1 and familial hypocalciuric hypercalcemia.