MEFs from E13.5-14.5 C57BL/6 mice were prepared as preciously described (Nam et al., 2014) and cultured in fibroblast growth medium (DMEM containing 10% fetal bovine serum (FBS) and 1% penicillin/streptomycin) until experiments were performed. MEFs infected were induced using DMEM/199 (4:1), 10% FBS, 5% horse serum, 1% penicillin/streptomycin, 1% nonessential amino acids, 1% essential amino acids, 1% B-27, 1% insulin-selenium-transferrin, 1% vitamin mixture, and 1% sodium pyruvate (Invitrogen). The induction medium was replaced every other two or three days until experiments were performed. MEFs two or seven days after retroviral transduction were crosslinked with 1% formaldehyde in PBS for 30 min and neutralized by the addition of glycine to a final concentration of 0.125M for 5min. MEFs were then harvested and washed with cold PBS for ChIP. For mouse heart sample preparation, ChIP was performed as previously described (Huang et al., 2016; Xu et al., 2012), or by using ChIP-IT Express ChIP kits (Active Motif) following the vender's protocol. ChIP-seq libraries were generated using KAPA Hyper Prep Kit following the manufacturer's protocol (Kapa Biosystems).