library_construction_protocol: The forelimbs of embryos were dissected into proximal and distal domains, then placed into PBS with 10% FCS and 8ul collagenase at 50mg/ml (Sigma C9697) at 37º for approximately 5min. Cells were centrifuged at 300xg for 5 minutes and the supernatant was removed. Cells were then resuspended in ice-cold 1xPBS with PIC and centrifuged again. The cells were then fixed in 1% PFA at room temperuatre for 10 minutes. A final wash was performed and cells were frozen at -80ºC until processing for Hi-C. Hi-C libraries were processed with the SureSelect XT library preparation kit