However, as CD25 expression is not restricted to Tregs, but can also be expressed by effector T cells, it was not until the discovery of FOXP3 as a unique marker of Tregs (Fontenot et al. 2003, Hori et al. 2003) and the development of reliable monoclonal antibodies that the presence of Tregs could be convincingly demonstrated in human cancers (Bates et al. 2006, Roncador et al. 2005). The gene discussed is FOXP3; the disease is cancer.