How T cells maintain their identity 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 (T _regs ) 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 T _regs . Using genetically modified mice, they found that both CD4 ⁺ and CD8 ⁺ T _regs require the transcription factor Helios to stably maintain their identity. Science , this issue p. 334