RB1 and osteosarcoma: The Osx-Cre transgenic mouse was chosen to conditionally delete Rb1 and Pten in these studies because Osterix is expressed in committed osteoprogenitor cells and because Osx-Cre has successfully been utilized to generate osteosarcoma mouse models that co-deleted Rb1 and the p53 tumor suppressor genes [6, 27, 28].