The deterioration of CD4 T-cell responses is believed to be responsible for the EBV-induced infectious mononucleosis that is seen in patients with X-linked lymphoproliferative disease, who have a mutation or deletion in signaling lymphocyte activation molecule (SLAM)-associated protein (SAP), an inhibitor of the T cell costimulatory molecule, SLAM or CDw150 [17]. Here, SH2D1A is linked to infectious mononucleosis.