“X-linked immunodeficiency with magnesium defect, Epstein-Barr virus (EBV) infection, and neoplasia” (XMEN) disease is an inborn error of glycosylation and immunity caused by loss of function mutations in the magnesium transporter 1 (MAGT1) gene. The gene discussed is MAGT1; the disease is X-linked immunodeficiency with magnesium defect, Epstein-Barr virus infection and neoplasia.