All mammalian genomes encode one or more APOBEC3 (A3) genes, cytidine deaminases (CDAs) that are intrinsic anti-viral restriction factors, inhibiting infection by a diverse array of viruses including retroviruses, parvoviruses, hepadnaviruses, and papillomaviruses and perhaps even herpes viruses. While these CDAs are critical to anti-viral immunity, they also have the potential to act on genomic DNA and thus their expression may come at a cost to genomic integrity. Indeed, there is accumulating evidence that A3 proteins act on cellular DNA, resulting in mutations. Our lab pioneered the use of A3 knockout mice to study their role in endemic retrovirus infection and have recently created human A3A and 3G transgenic mice that express functional protein in vivo. We propose to use wild type, knockout, and transgenic mice to directly test if genomic DNA damage or mutation occurs in vivo and whether such mutations have consequences for longevity and age-related diseases such as cancer. We will study these processes in wild type, knockout and transgenic mice, as well as lacZ mutator mice, which allow facile evaluation of non-lethal mutation rates in different tissues. Additionally, we will determine whether the mouse and human A3 proteins cause mutations in the APC tumor suppressor gene frequently mutated in human colorectal cancer, by crossing the KO and transgenic mice with APCmin mice. We expect that mutations will accumulate with age at a more rapid rate in APOBEC+ mice, particularly those expressing human A3 proteins, than in A3 knockout mice and that they will show characteristic signatures of CDA-mediated mutations. These mice afford us the unique ability to monitor the spectrum of mutations that arise in different tissues and will provide us with a model to directly test whether A3 proteins' potential ability to cause DNA mutations and damage in vivo has consequences for long-term health. It is particularly important to know whether A3 mediates genomic damage, given the association between the expression of A3 genes in particular human cancers, as well as the focus on targeting A3 activity as potential anti-viral therapies in humans. This study could establish a new area of aging research and since A3 genes are believed to function predominantly if not solely in antiviral immunity, it would be feasible to consider targeted therapies that limit their ability to cause genomic DNA mutation.