T he calcium-sensing receptor (CaR) is a G-protein-coupled receptor that plays a key role in extracellular calcium ion homeostasis. We have engineered 11 CaR mutants that have been described in the disorders familial benign hypercalcemia (FBH), neonatal severe hyperparathyroidism (NSHPT), and autosomal dominant hypocalcaemia (ADH), and studied their function by characterizing intracellular calcium [Ca2+]i transients in response to varying concentrations of extracellular calcium [Ca2+]o or gadolinium [Gd3+]o. The wild type receptor had an EC50 for calcium (EC50[Ca2+]o) (the value of [Ca2+]o producing half of the maximal increase in [Ca2+]i) of 4.0 mM (+/- 0.1 SEM). However, five missense mutations associated with FBH or NSHPT, (P55L, N178D, P221S, R227L, and V817I) had significantly higher EC50[Ca2+]os of between 5.5 and 9.3 mM (all P < 0.01). Another FBH mutation, Y218S, had an EC50[Ca2+]o of > 50 mM but had only a mildly attenuated response to gadolinium, while the FBH mutations, R680C and P747fs, were unresponsive to either calcium or gadolinium. In contrast, three mutations associated with ADH, (F128L, T151M, and E191K), showed significantly reduced EC50[Ca2+]os of between 2.2 and 2.8 mM (all P < 0.01). These findings provide insights into the functional domains of the CaR and demonstrate that mutations which enhance or reduce the responsiveness of the CaR to [Ca2+]o cause the disorders ADH, FBH, and NSHPT, respectively.