Interestingly, the fundamental significance of magnesium in immunity was unveiled after the characterization of XMEN (X-linked immunodeficiency with magnesium defect, Epstein–Barr virus infection, and neoplasia), a primary immunodeficiency due to a genetic deficiency of the transporter MAGT1 [191,192,193], which suggested that magnesium might function as a second messenger in cellular immune signaling. The gene discussed is MAGT1; the disease is inborn error of immunity.