library_construction_protocol: 100,000 CD8 T cells were obtained from spleen/lymph nodes of mice and were activated with anti-CD3/anti-CD28 and treated as indicated. Nuclei were isolated and Tn5 transposition was performed per manufacturer's instructions (Active Motif ATAC-Seq Kit). Libraries were PCR-amplified with Nextera-compatible i5/i7 primers. Libraries which passed QC (Bioanalyzer or Tapestation) were sequenced on Illumina NextSeq 2000 by the MSKCC Integrated Genomics Core.