In a bid to better understand the gene expression patterns that control T cell activity, researchers at the La Jolla Institute for Allergy and Immunology mapped genome-wide changes in chromatin accessibility as T cells respond to acute and chronic virus infections. Their findings, published in the Dec. 20, 2016 issue of Immunity, shed light on the molecular mechanisms that determine the fate of T lymphocytes and open new approaches to clinical intervention strategies to modulate T cell activity and improve immune function.