Nine days after the initial isolation and stimulation, 15000 resting Treg cells per sample were resuspended in ATAC Lysis Buffer (10mM Tris-HCl pH7.4, 10mM NaCl, 3mM MgCl2, 0.1% IGEPAL). Nuclei subjected to tagmentation using Nextera DNA Library Preparation Kit (Illumina). Tagmentation DNA was purified with the MinElute PCR Purification Kit (Qiagen #28004) and amplified with Phusion High-Fidelity PCR Master Mix (NEB #F531L) using 16 PCR cycles. Amplified libraries were re-purified.