Briefly, nuclei were isolated from 50,000 FACS sorted RFP+ Treg cells from reporter mice using a solution of 10 mM Tris-HCl, 10 mM NaCl, 3 mM MgCl2, and 0.1% IGEPAL CA-630. Immediately following nuclei isolation, the transposition reaction was conducted using Tn5 transposase and TD buffer (Illumina) for 45 min at 37o C. Transposed DNA fragments were purified using Qiagen MinElute Kit and PCR amplified using NEBNext High Fidelity 2x PCR master mix (New England Labs) with dual indexes primers (Illumina Nextera). The size distribution and molarity of the sequencing library were determined by using an Bioanalyzer analysis (High Sensitivity DNA chip, Agilent).