Chronic graft-versus-host disease (GVHD), which is induced in (C57BL/6 × DBA/2) F1 (BDF1) mice by injection of DBA/2 spleen cells, is associated with the activation of donor CD4+ T cells that recognize host major histocompatibility complex (MHC) antigens and drive host B cell hyperactivity [4,5]. The gene discussed is CD4; the disease is chronic graft versus host disease.