library_construction_protocol: E14.5 Meckel's cartilage bars were dissected from wild-type C57BL/6J  mouse embryos as previsouly described (Ishizeki, 1996 & Wiszniak, 2015) with modifications. E14.5 forelimb was dissected from wild-type mouse embryos with removal of surrounding connective tissues. Dissected forelimb cartilage was then digested following the same procedure as the digestion of Meckel's cartilage. ATAC-seq was performed as described previously with TruePrep DNA Library Prep Kit V2 for Illumina (Vazyme TD501). Briefly, 50,000 cells were collected at 500×g for 5 min and resuspended in 50μl cold ATAC-seq lysis buffer and incubated on ice for 10min. Following centrifugation at 500×g at 4°C for 5 min, cells were pelleted and resuspended in 50μl transposition mix and then incubated at 37°C. The reaction was purified with VAHTS DNA Clean Beads and PCR amplified for 15 cycles. Purified ATAC-seq libraries were sequenced on the NovaSeq platform.