Just before sampling the embryos for DNase-seq, we removed the polar bodies to avoid genomic contamination from polar body. First, the embryos were briefly exposed to the Acid Tyrode's solution (Sigma-Aldrich) supplemented with 0.5% polyvinylpyrrolidone (PVP, Irvine Scientific, 90123) and 50 mM NaCl to gently remove zona pellucida. After washing with M2 media (Millipore), the embryos were incubated with Trypsin/EDTA (Life technologies) supplemented with 0.5% PVP and 50 mM NaCl for 2-3 min on multitest glass slide (MP Biomedicals, 6040805). The embryos were then washed with M2 media followed by washing with PBS with 0.2% BSA and transferred into an Eppendorf LoBind 1.5 ml tube (Eppendorf) on ice. Sequencing library was prepared using NEBNext Ultra DNA Library Prep Kit for Illumina (New England Biolabs) with the exception that PCR amplification was done using Kapa hifi hotstart readymix (Kapa Biosystems) for 8-cycles. The PCR product was purified with 1.3 volume SPRIselect beads (Beckman Coulter) and then size selected with 0.7 volume plus 0.7 volume SPRIselect beads. Then the sample was amplified again with Kapa hifi hotstart readymix for 6 to 11 cycles. Repeat the previous purification and size selection step. The libraries were sequenced on a Hiseq2500 with single-end 100 bp reads (Illumina).