CASR and Hypomagnesemia: Abnormalities of the calcium‐sensing receptor (CaSR); Familial hypomagnesemia hypercalciuria syndrome (CLDN16 mutation); other claudin family of genes (particularly CLDN10, CLDN14, CLDN19) mutation; Familial hyperkalemic hypertension (WNK1 or WNK 4 mutation); NHE3 mutation