Even though they account for just 5% of the total CD4 T cell population, it is well established that Tregs (CD4+CD25+Foxp3+) are essential for controlling both acute and chronic GVHD following allo-HSCT, and several studies support the inverse correlation between circulating Treg levels and GVHD occurrence [70,71,72,73,74]. The gene discussed is FOXP3; the disease is graft versus host disease.