ATAC-seq: 50,000 freshly sorted cells from differentiated ES cultures, iPax3 NIH3T3 fibroblasts and single embryos were washed with 200 µl of cold PBS then resuspended in 100 µl of cold lysis buffer (10mM Tris-HCl pH 7.4, 10mM NaCl, 3mM MgCl2, 0.1% IGEPAL CA-630), spun at 500 g for 10 minutes at 4°C and resuspended in 50 µl of the transposition reaction mix. Transposition occurred at 37°C for 30 minutes, after which transposed DNA was purified using a Qiagen MinElute Kit and eluted in 10µl Elution Buffer. Final libraries were generated by primer extension of the transposed DNA followed by PCR amplification using Illumina-compatible adapter-barcodes.