Gammaherpesviruses (GHVs) establish lifelong chronic infections that place the host at risk for numerous cancers. During chronic infection, GHVs express viral gene products that stimulate host-cell proliferation and differentiation, processes thought to facilitate long-term latent persistence and contribute to tumorigenesis. However, GHVs are not acutely transforming, and cancer is rare given the high incidence of infection among adult humans, estimated at more than 90% for Epstein-Barr virus. This suggests that host cells are equipped with an intrinsic resistance to GHV-driven proliferation and cellular immortalization, but few host molecules capable of mediating this effect are known and whether these molecules functionally restrict GHV persistence and disease in vivo has not been established. Experiments described in this proposal seek to identify and confirm the in vivo relevance of host molecules with the capacity to limit GHV chronic infection and disease in three Specific Aims. Based on preliminary studies that employed murine gammaherpesvirus-68 (MHV68) infection of mice, experiments described in Specific Aim 1 test the function of host tumor suppressor protein p53, a critical molecule for preventing neoplastic disease, as an innate barrier to MHV68 latent persistence and cellular transformation. Experiments presented in Specific Aim 2 seek to define mechanisms whereby p53 limits MHV68 latent infection. Experiments in Specific Aim 3 will provide the first in vivo test of the long- standing hypothesis that latency-associated nuclear antigen, a conserved GHV disease determinant capable of inhibiting p53 functions in tissue culture, is necessary to overcome a p53-mediated restriction to enable long- term latent infection. These experiments harness the powerful mouse and MHV68 genetic systems to address a question that is fundamental to our understanding of the GHV-host dynamic. Further, we anticipate that results of this work will inform new therapeutic approaches aimed at enhancing p53 functions to limit or prevent GHV-related cancers, especially in high-risk patients such as HIV-infected individuals or solid-organ transplant recipients.