PROJECT SUMMARY/ABSTRACT Intestinal cancer is the 3rd most common malignancy and cause of cancer-related deaths in both men and women. Unfortunately, recent advances in our understanding of the underlying intestinal biology and cancer- related pathways have not translated to significantly improved patient outcomes. While numerous animal models of intestinal cancer have been developed, mostly based on misregulation of the β-catenin pathway, some key features of human cancers (particularly tumor mutational burden) now known to be critical for tumor progression and therapy response, have not been adequately modelled or investigated in animal models of intestinal cancer. More recently, genome sequencing efforts led to the discovery of an intestinal cancer mutator phenotype where single amino acid substitutions within proofreading domains of the housekeeping DNA polymerases result in the highest mutation rates described in human cancers (ultramutation). Unlike human cancers, genetically-engineered animal models exhibit very low mutation rates, limiting their utility for studies of intratumoral heterogeneity and competition, immune responses, and immune checkpoint therapies, now known to be essential aspects of human tumor biology. We propose to overcome these limitations in intestinal cancer animal models by building upon 1) a strong track record in the generation of cancer animal models and novel genetic tools for their development 2) a well-characterized conditional PoleP286R allele that we previously used to develop a robust model of endometrial cancer and 3) expertise in genomics, inflammation, intestinal cancer, and mouse models of intestinal disease. These models will be useful not only to recapitulate POL-driven intestinal cancers, but also to humanize any intestinal cancer mouse model with respect to mutational burden. This proposal is submitted in response to PAR-20-131 to expand and improve the development of mammalian models for translational cancer research.