MAGT1 and severe combined immunodeficiency: “X-linked immunodeficiency with magnesium defect, Epstein-Barr virus (EBV) infection, and neoplasia” (XMEN) disease is a combined immunodeficiency (CID) caused by hemizygous loss of function (LOF) mutations in the magnesium transporter 1 gene (MAGT1) located on the X chromosome [1–4].