Abstract Context Several heterozygous loss-of-function mutations in the calcium-sensing receptor gene (CASR) leading to elevated ionized serum calcium and familial hypocalciuric hypercalcemia 1 (FHH1) have been characterized. Few mutations are not pathogenic, and previous studies suggested that the Q459R mutation does not result in an FHH1 phenotype. Objective We identified a family with a heterozygous CASR Q459R mutation and characterized their calcium homeostasis and the pathophysiological mechanisms of a homozygous and heterozygous Q459R mutation in vitro. Design The index patient and her family had clinical, biochemical, and genetic analyses performed. In vitro functional characterization of homozygous and heterozygous (Q459R) mutations was conducted by determining CaSR cell-surface expression and inositol monophosphate (IP1) signaling in transiently transfected human embryonic kidney 293A (HEK293A) cells. Results All 3 heterozygous carriers had mild asymptomatic hypercalcemia, hypocalciuria, and 2 had elevated serum parathyroid hormone (PTH). In vitro characterization in HEK293A cells revealed that CASR Q459R is a loss-of-function mutation with no impact on cell-surface expression. Cells with the homozygous Q459R genotype had significantly reduced calcium potency of IP1 signaling compared to wild type, whereas the heterozygous Q459R also had lower calcium potency albeit not significantly different from wild type. Conclusion A loss-of-function Q459R mutation in CASR in a family caused FHH1 characterized by elevated ionized calcium and PTH and low calcium excretion. The marked presence of CaSR at the membrane and inhibition of IP1 signaling in vitro suggest that calcimimetics may be functional in patients with this mutation, which seems to be a mild loss-of-function mutation associated with autosomal dominant transmission of FHH1.