1-3 million of feeder-independent mESCs were harvested for genomic DNA extraction by using a QIAamp DNA Mini Kit (QIAGEN, 51306). Plasmid DNA were extracted by using a NucleoBond Xtra BAC kit (Takara, 740436.25) Approximately 100 ng DNA was used for the library construction using the NEBNext Ultra II FS DNA library prep kit (E7805). AMPure XP beads (Beckman Coulter, A63881) were used for DNA purification on a magnetic stand. DNA libraries were loaded on a ZAG DNA analyzer (Agilent) for quality control.