CLDN19 and familial primary hypomagnesemia with hypercalciuria and nephrocalcinosis: Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is an ultra-rare autosomal recessive renal tubular disease with an incidence of less than 1 in 1,000,000 individuals, caused by loss-of-function mutations in CLDN16 and CLDN19.