FHHNC patients harbor homozygous or compound heterozygous mutations in the CLDN16 (OMIM248250) or CLDN19 genes (OMIM248190), which encode the tight junction structural proteins claudin-16 [1] and claudin-19 [2], respectively. Here, CLDN16 is linked to familial primary hypomagnesemia with hypercalciuria and nephrocalcinosis.