TP53 and neoplasm: To confirm that the tumors originated from epithelial cells, we harnessed the flexibility of the EPO-GEMM approach to directly mutagenize Cytokeratin-8 (CK8)-expressing epithelial cells, a cell type that can serve as a tumor-initiating cell in the absence of Trp53 (34), and is also retained in traditional GEMMs arising in Brca1;Trp53;Pten-deficient mice (24).