CTL cells were isolated using anti-CD45 monoclonal antibody-conjugated magnetic bead. Nuclei were prepared from cultured cell lines using the Omni-ATAC protocol Nat Methods. 2017 Oct; 14(10): 959–962.) For each sample, 50,000 nuclei prepared from cultured cells were incubated with 2.5 mL Nextera Tn5 transposase (Illumina) in 50mL 1X transposition reaction mixture in a thermocycler at 37 °C for 1hr. The transposition reaction mixtures were purified with a DNA Clean and Concentrator kit (Zymo Research), and amplified for 11-13 cycles using NEBNext High-Fidelity 2X Master Mix (New England Biolabs) and Nextera Index primers (Illumina). The amplified libraries were size-selected by two rounds of AMPure beads purification (0.5X and 1.2X).