When hypercalcaemia is associated with inappropriately detectable PTH (within laboratory “normal” range or elevated), then causes to consider include rare inactivating (loss-of-function) CASR gene mutations which result in CaSR insensitivity and thus PTH secretion is not switched-off until higher-than-normal calcium concentrations, causing Familial Hypocalciuric Hypercalcaemia (FHH). This evidence concerns the gene CASR and familial hypocalciuric hypercalcemia.