The histological characteristics of the fibrosarcomas, including overexpression of Pdgfra and high grade features such as pleomorphic nuclei and numerous mitotic figures, were virtually identical to tumors that were previously described in detail in a PdgfrαK/+; INK4A/Arf−/− tumor model with general Cre activation18. The gene discussed is PDGFRA; the disease is fibrosarcoma.