OT-I cell subsets were isolated from spleen, purified by FACS and DNA was extracted from 50,000 OT-I cell subsets using cold lysis buffer (as described by Buenrostro et al. 2015). Nuclei pellets were resuspended in the transposition reaction mix (Nextera Kit) and incubated at 37°C for 30 min. Qiagen MinElute PCR Purification Kit was used to purify DNA. To amplify transposed DNA fragments, PCR with barcoded PCR primers (Buenrostro et al., 2013) and NEBNext High-Fidelity 2x PCR Master Mix was performed. To reduce GC and size bias in PCR, the appropriate number of PCR cycles was determined using qPCR. The amplified library was purified using a Qiagen MinElute PCR Purification Kit.