SOX9 and osteoporosis: MSX1 and PITX1 regulons were enriched in LBM1 and LBM2,65,66 while PBX1 and SOX9 regulons were enriched in LBM3 and OCP.22,67 Interestingly, we also identified several OCP-specific regulons such as ZMIZ1 and KDM5A (Fig. 2i; Supplementary information, Table S2), which were shown to play important roles in neurodevelopmental disorder68 and osteoporosis,69 suggesting novel osteo-chondrogenic regulators within the limb bud mesenchyme.