As mentioned, the functional importance of Mg transport in immunity was put in evidence with the discovery of the XMEN disease that presents with defective T lymphocyte activation and chronic Epstein–Barr virus infection due to a genetic deficiency of MAGT1 and showing for the first time that Mg could function as a second messenger in cellular signaling [12,13,14]. The gene discussed is MAGT1; the disease is X-linked immunodeficiency with magnesium defect, Epstein-Barr virus infection and neoplasia.