PRRX1 and osteosarcoma: Prrx1-CreERT;Ptch1f/f mice also developed tumors at the periosteal surfaces that have features of osteosarcoma: expansive osteoid lesions with mushroom-shaped appearance that were only located at cortical bones of the limbs (Figure 3A–3D), which later transgressed the cortex (Figure 3—figure supplement 1A).