CD4 and graft versus host disease: In recent years, the great progresses in our understanding of the basic processes that control immune tolerance, as well as the more recent characterization of naturally arising CD4+CD25+Foxp3+ regulatory T cells (Treg), that tip the balance between auto- and tumor immunity, opened the door to their therapeutic application, either by enhancing their activity in autoimmune diseases [1–3], allograft rejection [3], and graft-versus-host disease (GVHD) [4, 5] or by blocking their suppressive activity in tumor immunity [6] and in vaccine development [7].