Early studies on XMEN patients suggested that loss of MAGT1 resulted in a decreased free intracellular Mg2+ concentration [Mg2+] and impaired T-cell receptor (TCR)-induced transient Mg2+ influx, but the specificity of the fluorescent Mg2+ indicators has been questioned [1, 2]. This evidence concerns the gene MAGT1 and X-linked immunodeficiency with magnesium defect, Epstein-Barr virus infection and neoplasia.