In sharp contrast to the previous findings that the Krasex3op allele reduced urethane carcinogenesis (Pershing et al., 2015), the loss of p53 instead increased tumor burden in mice with at least one Krasex3op allele (Figure 1B), similar to what was observed in a whole animal Cdkn2a null background (Pershing et al., 2015). This evidence concerns the gene TP53 and neoplasm.