AP2S1 and familial hypocalciuric hypercalcemia: FHH is genetically heterogeneous and comprises three distinct variants, designated FHH types 1–3, which are caused by loss-of-function mutations of the CASR, guanine nucleotide-binding protein subunit alpha-11(GNA11), and adaptor protein complex-2 subunit sigma (AP2S1) genes, respectively (Fig. 1).