RB1 and osteosarcoma: We found that deletion of Trp53 and Rb1 in Ctsk‐expressing cells in mice caused osteosarcoma formation (Figure 1A); however, single deletion of either Trp53 or Rb1 in Ctsk‐expressing cells (Ctsk‐Cre;Trp53f/f and Ctsk‐Cre;Rb1f/f) did not lead to osteosarcoma formation at 2, 14, or 18 months (Figure S1C).