The seminal work of Shimon Sakaguchi’s team (7) described a new subset of CD4+ T cells expressing the IL-2 receptor α chain (CD25) that when “eliminated” “produces a wide spectrum of organ-specific autoimmune diseases, systemic autoimmunity, and GVHD-like wasting disease in normal mice.” The study also demonstrated that adoptive transfers of CD4+ CD25+ T cells prevented these autoimmune developments. Here, CD4 is linked to graft versus host disease.