FHH3 results from the inactivating mutation of the AP2S1 gene that encodes the adaptor-protein 2 sigma subunit, a scaffolding protein that is essential in CaSR endocytosis, and this mutation is found in approximately 20% of FHH patients without CaSR mutations [9]. The gene discussed is AP2S1; the disease is familial hypocalciuric hypercalcemia.