Mutations in CLDN16 and CLDN19, which encode the tight junction proteins claudin-16 and claudin-19 respectively, give rise to the condition familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) [29]. The gene discussed is CLDN19; the disease is familial primary hypomagnesemia with hypercalciuria and nephrocalcinosis.