Because FOXP3 expressing Treg cells represent 1 to 6 % of CD4+ T cells in humans [9], in vitro expansion strategies are required to enable the infusion of significant number of Treg cells, given that 2 million Treg cells are usually required for infusion in mice (105 Treg cells per gram) to prevent autoimmunity [10]. The gene discussed is FOXP3; the disease is Autoimmunity.