The functional importance of Mg transport in immunity was unknown until the description of a new primary immunodeficiency named XMEN (X-linked immunodeficiency with Mg defect, Epstein–Barr virus infection, and neoplasia) due to a genetic deficiency of MAGT1 [12,13,14] suggesting that Mg could function as a second messenger in cellular signaling. The gene discussed is MAGT1; the disease is neoplasm.