Loss-of-function mutations in the calcium-sensing receptor (CaSR) gene (FHH type-1) [1], guanine nucleotide-binding protein subunit alpha 11(GNA11) gene (FHH type-2) [5, 6], and adaptor-related protein complex 2 sigma subunit 1 (AP2S1) gene (FHH type-3) [7–10] are responsible for most but not all FHH cases [11, 12]. This evidence concerns the gene AP2S1 and familial hypocalciuric hypercalcemia.