ATAC-seq (Assay for Transposase-Accessible Chromatin using sequencing) was performed as previously described (Buenrostro et al 2015, Curr Protoc Mol Bio). Nuclei were isolated from micro-dissected tissue from E9.5 CD-1 mouse embryos (2x from 5 embryo pools each) from Charles River according to published protocols and transposition reaction performed by published methods (Buenrostro et al 2015, Curr Protoc Mol Bio). Libraries were amplified and normalized with the Illumina Nextera DNA Library prep kit (FC-121–1031) according to the manufacturer's protocols. Libraries were quantitated using the Agilent Bio-analyzer, pooled in equimolar amounts, and sequenced with 50-bp single-end reads on the Illumina HiSeq following the manufacturer's protocols through the Genomics Core Facility at the University of Chicago.