ATAC-seq from two biological replicates was performed essentially as previously described (Buenrostro et al., 2015) with the following differences: in total, 2,000-5,000 embryonic cells were used per ATAC-seq library and the transposition reaction was done in 5 ul instead of 50 ul reaction. Also, the QIAGEN MinElute purification before PCR was eliminated and instead took the 5 ul reaction immediately after transposition directly into the 50 ul PCR. The ATAC-seq libraries were quantified using the NEBNext Library Quant Kit for Illumina (NEB, #E7630S) and the size distribution of the libraries was validated using the High Sensitivity DNA Analysis Kit (Agilent, #5067-4626). The libraries were pooled and paired-end sequenced on an Illumina NextSeq 500 with 2x38bp read lengths.