CLDN16 and familial primary hypomagnesemia with hypercalciuria and nephrocalcinosis: In this case report, we describe a new homozygous missense mutation on <i>CLDN16</i> gene (c.592G>C, Gly198Arg) in a 24-year-old male patient diagnosed with FHHNC after clinical investigation due to incidental detection of altered routine laboratorial tests, who was firstly misdiagnosed with primary hyperparathyroidism.