Transcription factors define tissue T cells The immune system fights microbial invaders by maintaining multiple lines of defense. For instance, specialized memory T cells [resident memory T cells (T _rms )] colonize portals of pathogen entry, such as the skin, lung, and gut, to quickly halt reinfections. Mackay et al. now report that in mice, T _rms as well as other tissue-dwelling lymphocyte populations such as natural killer cells share a common transcriptional program driven by the related transcription factors Hobit and Blimp1. Tissue residency and retention of lymphocytes require expression of Hobit and Blimp1, which, among other functions, suppress genes that promote tissue exit. Science , this issue p. 459