The origins of mutagenesis that result in cancer development and progression are largely unknown. The APOBEC3 cytidine deaminases are a suspected source of mutagenesis in cancer and are associated with poor outcomes, however their impact on tumor development and behavior is unknown. We are investigating how APOBEC3 activity influences malignant transformation, and the subsequent effect of deamination on tumor biology. We use various cellular and animal models to study mutagenesis in leukemia and solid tumors.