FOXP3 is a specific marker for naturally occurring regulatory T cells (nTreg). FOXP3 expression is correlated with development and function of nTreg. Recent evidence suggests that, upon activation, human effector T (Teff) cells and type 1 regulatory T (Tr1) cells can express FOXP3, albeit transiently. While the role of transient FOXP3 expression in Teff cells is poorly understood, it is becoming clear that it does not correlate with suppressor function. Furthermore, FOXP3-independent mechanisms, mediated by IL-10, contribute to the induction and suppressor functions of Tr1 cells.