Although best known for their pathogen-fighting prowess, T lymphocytes also ensure that the immune response does not run amok. A subset of T cells called regulatory T cells (Tregs) performs this function by, for example, making sure T cells only attack pathogens and not self. T cells can exhibit plasticity in their functions in the face of an inflammatory stimulus. Kim et al. sought to identify the molecules that ensure the stable maintenance of Tregs. Using genetically modified mice, they found that both CD4+ and CD8+ Tregs require the transcription factor Helios to stably maintain their identity.
