Loss‐of‐function mutations in two genes involved in renal paracellular reabsorption of calcium and magnesium, CLDN16 (OMIM *603959) and CLDN19 (OMIM *610036), cause two types of a rare autosomal recessive disease known as Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis, respectively (FHHNC; type 1, OMIM #248250 and type 2, #248190) (Konrad et al., 2006; Simon et al., 1999). The gene discussed is CLDN16; the disease is familial primary hypomagnesemia with hypercalciuria and nephrocalcinosis.