Cells were harvested, counted and centrifuged for 3 min at 600×g at room temperature.20,000 cells were lysed with cold lysis buffer (10 mM Tris-HCl, pH 7.4, 10 mM NaCl, 3 mM MgCl2 and 0.03% Tween-20), and centrifuged at 500 g for 8 min at 4 °C. The supernatant was carefully removed, and the nuclei was resuspended with Tn5 transposase reaction mix (25 μl 2 × TD buffer, 2.5 μl Tn5 transposase and 22.5 μl nuclease-free water) (Illumina) at 37 °C for 30 min. Immediately after the transposition reaction, DNA was purified using a Qiagen MinElute kit. Libraries were constructed using TruePrep DNA Library Prep Kit V2 for Illumina (Vazyme, China) under manufacturer's instructions and were then sequenced on an Illumina HiSeq X Ten sequencer to an average read depth of 52 million pair-end reads per sample. The ATAC-seq experiment and library sequencing were performed by Frasergen Bioinformatics Co., Ltd, Wuhan, China. ATAC libraries were constructed using TruePrep DNA Library Prep Kit V2 for Illumina (Vazyme, China) under manufacturer's instructions and were then sequenced on an Illumina HiSeq X Ten sequencer to an average read depth of 52 million pair-end reads per sample.