KRAS and teratocarcinoma: Mouse embryonic fibroblasts (MEFs) with conditional Pten and Kras alleles were reprogrammed and then infected with a Cre-expressing adenovirus (Ad-Cre) to inactivate Pten and constitutively activate Kras. We anticipated that without targeting Pten and Kras the iPS cells would form benign teratomas, whereas the transformed cells would generate malignant teratocarcinomas.