Associations with serum potassium and sodium did not reach the level of genome-wide significance in our study, but common genetic variants in six genomic regions in or near the MUC1, MDS1, SHROOM3, TRPM6, DCDC5, and ATP2B1 genes were significantly and reproducibly associated with serum magnesium levels and clinically defined hypomagnesemia. The gene discussed is MUC1; the disease is familial primary hypomagnesemia.