library_construction_protocol: EBs were collected at different time points and disaggregated in 0.25% trypsin at 37°C for 3 min incubation with gentle agitation followed by inactivation with culture medium containing 10% FBS. Cells were collected by centrifugation at 500g for 5 minutes, washed once with ice-cold PBS. iHA-Etv2 MEFs were harvested from the culture dishes at different time points by treating the cells with 0.25% trypsin at 37°C for 4 min incubation followed by neutralizing and collecting the cells with culture medium containing 10% FBS. ATAC reaction was performed with 50,000 cells as previously described18 using the Tn5 transposase (Illumina) and libraries were created at the University of Minnesota Genome Center. Libraries were then sequenced on a NextSeq Illumina platform (2x50 bp) aiming for 25 million reads per sample. ATAC-seq libraries were prepared for sequencing using standard Illumina protocols