COL1A1 and neoplasm: After 8 weeks of transplantation, Fgfr3CE-P21∆p53 clones robustly differentiated into Col1a1-GFP+ osteoblasts on the bone surface and generated highly-trabecularized structures throughout the marrow space reminiscent of early-stage osteosarcoma-like lesions, associated with tumor cells with atypical nuclei and endocortical resorption with scalloping patterns (Fig. 9f–h).