Patients with acute GVHD (aGVHD) and chronic GVHD (cGVHD) show deficits of CD4+CD25high regulatory T (Treg) cells, which express the transcription factor, fork-head box P3 (FOXP3), and play a crucial role in control of autoimmunity [2, 3]. The gene discussed is FOXP3; the disease is Autoimmunity.