Cell nuclei were fresh prepared from 50,000 cells as previously described (Buenrostro et al. 2013 Nature Methods). ATAC-seq was performed as previously described (Buenrostro et al. 2013 Nature Methods). For each sample, cell nuclei were prepared from 50,000 cells, and incubated with 2.5 uL transposase (Illumina) in a 50 uL reaction for 30 min at 37°C. Following purification of transposase fragmented DNA, the library was amplified by PCR and subjected to high-throughput sequencing using Illumina HiSeq 2500 platform.