library_construction_protocol: After harvest and washing with cold PBS, 50,000 cells were gently lysed in 50μl of cold lysis buffer before centrifugation to yield a nuclear pellet nuclei were treated with the transposition reaction mix at 37°C for 30 min (Nextera Tn5 Transposase) before purification and elution into 10μl elution buffer (Buffer EB 10 mM Tris·Cl, pH 8.0; Qiagen MinElute PCR Purification Kit). Lastly, adapters were added prior to PCR-based library amplification. Library quality control was performed using gel electrophoresis and the libraries pooled at equimolar ratios with barcodes and sequenced on the BGISEQ-500 platform (BGI-Shenzhen, China).