When the immune system encounters an antigen, the response can result in the mobilization of effector cells or in tolerance. The outcome is largely dependent on immunosuppressive CD4 T cells that express the transcription factor Foxp3 (Tregs). Yet, how Tregs control different immune effector cells remains elusive. In this issue of The EMBO Journal, Dhainaut et al report on a novel mechanism used by Tregs to prevent differentiation of naïve CD4 T cells to proinflammatory Th1CD4 (Th1) effectors.