Hereditary forms of PHPT account for approximately 10-15% of all cases and encompass syndromes such as MEN1, MEN2A, MEN4, MEN5, HPT-JT, familial hypocalciuric hypercalcemia (FHH), and familial isolated hyperparathyroidism (FIHP) (98). Here, CDKN1B is linked to familial isolated hyperparathyroidism.