Deletion of Trp53 and Rb1 in Prx1‐positive mesenchymal lineage cells causes osteosarcoma7; consistent with this previous finding, other studies have shown osteosarcoma development after loss of Trp53 and Rb1 in osteoblast precursors and osteocytes, indicating that osteogenic lineage cells may be the principal sources for osteosarcoma formation.8, 9, 10. This evidence concerns the gene RB1 and osteosarcoma.