RB1 and neoplasm: To examine how ATRX loss alters tumor initiation in OS, we chose to work with a previously established transgenic Osterix-Cre mouse model with conditional (floxed) alleles of both p53 (p53fl/fl) and Rb (Rbfl/fl) and with a Tet-off cassette providing an additional level of temporal control (36, 37).