Studies using murine MSC containing CRE-inactivated P53 and/or RB alleles also reveal relevant clues about the nature of the OS-initiating cell and the factors conditioning their sarcomagenic potential. P53−/− and P53−/−RB−/− adipose-derived-MSC (ASC) or BM-derived-MSC (BM-MSC) give rise to leiomyosarcoma-like tumors when injected subcutaneously into immunodeficient mice [72–74]. This evidence concerns the gene TP53 and leiomyosarcoma.