library_construction_protocol: Limb bud tissues were digested in 200 ml of TripLE (ThermoFisher 12605010) at 37°C in two cycles of 5 min incubation followed by trituration. Cells were cooled on ice for 5 min between incubations. Digestion was stopped with 1ml of medium with 10% fetal calf serum that was filtered through a 50 mm mesh filter (Partec). Cells were collected by centrifugation and stained with staining medium containing 0.2 mg each of CD31-APC, Tie2-APC, CD45-APC and 0.5 mg anti CD16/CD32, and EYFP positive, APC negative live cells were sorted on FACSAria (BD). Cells from EYFP negative embryos were used as a negative control for FACS sorting.  ATAC-seq libraries were prepared for sequencing using standard Illumina protocols