Recently, an ex vivo analysis demonstrated that both T cell receptor stimulation-induced CD4+ T cell proliferation and CD25+CD4+ Treg-mediated immunosuppression were unchanged, whereas costimulatory efficacy (verified in animal models) of circulating costimulatory cells decreased within the first three days after experimental and human ischemic stroke onset, indicating the decrease in circulating costimulatory cells may be responsible for stroke-induced immunosuppression [81]. The gene discussed is CD4; the disease is ischemic stroke.