library_construction_protocol: The forelimbs or forebrains of embryos were dissected and placed into PBS with 10% FCS and 8ul collagenase at 50mg/ml (Sigma C9697) at 37º for approximately 5min. Cells were counted and 50,000 cells were isolated for processing with the Nextera Tn5 enzyme (Illumina FC-131-1096) as previously described (Buenrostro NatMeth 2013). Tn5 treated DNA was amplified with Nextera Library primers using NEBNext library amplification master mix (NEB M0541).