Acute EBV infection (infectious mononucleosis) and X-linked lymphoproliferative disease involve EBV-infected CD21-positive B-cells, with the former typically exhibiting a mild, self-limited clinical course and the latter presenting with more severe illness and molecular evidence of a mutated SH2D1A gene [5]. The gene discussed is SH2D1A; the disease is Epstein-Barr virus infection.