AP2S1 and familial hypocalciuric hypercalcemia: AP2S1 mutations have been reported in 19 FHH patients and families to date, and these all comprise heterozygous missense substitutions of the AP2 σ2-subunit (AP2σ2) Arg15 residue (Arg15Cys, Arg15His and Arg15Leu) (11,12,14).