These peculiarities suggest that parathyroid cells are highly resistant to cancer, though parathyroid cells are stimulated by several conditions, whose persistence induces cell proliferation: (1) pregnancy increases the rate of PTH release in the first trimester to face the embryonic needs; (2) chronic calcium and vitamin D deficiencies induce benign proliferative cell response; (3) congenital inactivating mutations of the calcium sensing receptor (CASR) gene are associated with parathyroid glands hyperplasia. Here, CASR is linked to cancer.