CASR is highly expressed in the parathyroid glands: elevated calcium levels activate the receptor, inhibiting PTH secretion; on the contrary, decreasing blood calcium level inactivates the CASR receptor, stimulating PTH secretion within a few seconds, an increase in PTH synthesis within minutes and, in the case of chronic hypocalcaemia, parathyroid hyperplasia [11,12]. The gene discussed is PTH; the disease is parathyroid hyperplasia.