RB1 and sarcoma: Thus, the inactivation of P53 and/or RB in early mesenchymal progenitors of embryonic limb buds through PRX1-driven CRE expression (PRX1-CRE) resulted in sarcoma development, presenting an OS incidence of 60% in P53−/− mice and 20–30% in P53−/−RB−/− mice, where most of the alternate tumors formed poorly differentiated soft tissue sarcomas [14, 16].