Familial primary hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a form of familial primary hypomagnesemia (FPH), characterized by renal magnesium (Mg) and calcium (Ca) wasting, nephrocalcinosis, kidney failure and, in some cases, severe ocular impairment. Two subtypes of FHHNC are described: FHHNC with severe ocular involvement (FHHNCOI) and without severe ocular involvement (FHHN). The disease is Monarch Disease Ontology identifier MONDO_0017624 (familial primary hypomagnesemia with hypercalciuria and nephrocalcinosis). Also known as: FHHNC, Michellis-Castrillo syndrome.