CLDN16 and CLDN19 colocalize at the TJ in a subset of TAL cells (10, –12), where they permit the transport of the divalent cations Ca2+ and magnesium (Mg2+) (13) Consistently, autosomal recessive variants in the CLDN16 and CLDN19 genes give rise to urinary wasting of these cations, resulting in a severe syndrome: Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) (1, 14). Here, CLDN19 is linked to familial primary hypomagnesemia.