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. The gene discussed is PTEN; the disease is teratocarcinoma.