ECs were washed twice with cold PBS, and nuclei were isolated by resuspending ~5000 cells in 50 uL cold lysis buffer (10 mM Tris-HCl, 10 mM NaCl, 3 mM MgCl2, and 0.1% NP-40). The nuclear pellets were resuspended in 50 uL transposition buffer (Nextera DNA Library Preparation Kit and 22.5 uL nuclease free water, and then incubated at 37°C for 30 min. After incubation, the DNA samples were purified by the Qiagen MinElute PCR purification kit. Librariy preparation and Illumina sequencing were done by the IGM Genomics Core, University of California, San Diego.