The first human studies of Treg cell therapy explored its potential in the regulation of GVHD and tested HLA-matched sibling donor-derived expanded CD4+CD25+CD127low/− (second generation) Treg to combat the difficulty in treating GVHD, following allogeneic bone marrow or stem cell transplantation (80, 81). The gene discussed is CD4; the disease is graft versus host disease.