Interestingly, genetically modified mice and patients with primary hyperparathyroidism are reported to have increased plasma FGF23 concentrations (28,29), although these have not been assessed in FHH patients to-date, and thus our findings from the Ap2s1+/L15 mice indicate that such measurements of FGF23 are warranted in FHH patients. Here, AP2S1 is linked to familial hypocalciuric hypercalcemia.