Several IEIs arise predominantly due to defects within the T cell compartment, thus correction of T cell populations alone may ameliorate clinical manifestations in conditions such as CD40 ligand deficiency, Immune dysregulation polyendocrinopathy enteropathy X-linked (IPEX) syndrome, X-linked lymphoproliferative syndrome (XLP), cytotoxic T-lymphocyte antigen 4 (CTLA4) insufficiency and signal transducer and activation of transcription 1 (STAT1) loss of function/gain of function mutations. The gene discussed is CD40; the disease is X-linked lymphoproliferative disease.