ATAC-seq libraries were prepared following the Fast-ATAC protocol (Corces et al., 2016). Briefly, frozen cells were thawed, washed in media, aliquoted and resuspended in Tn5 reaction buffer and incubated at 37℃ for 30 min. Transposed samples were purified using MinElute PCR purification columns according to the manufacturer's protocol (Qiagen, Germany). Purified samples were amplified and indexed using custom Nextera barcoded PCR primers as described in Buenrostro et al (Buenrostro et al., 2013). Amplified libraries were purified using MinElute PCR purification columns followed by gel-based size selection to remove free PCR primers.