Significant portions of mammalian genomes contain sequences that originate from ancient retroviral infections. These “endogenous retroviruses”no longer represent infectious viruses due to many mutations they have accumulated; yet these genetic sequences are important in health and disease. Endogenous retroviruses (ERVs) modulate reproduction, immunological processes, and are relevant in response to infection and during oncogenesis. One of the most well-documented ERVs, endogenous feline leukemia virus (enFeLV), represents a remnant of infectious feline leukemia virus (exFeLV) infection of cats. Some interactions between exFeLV and enFeLV have been well-studied; for example, when a cat is infected with exFeLV, there is a chance that exFeLV and enFeLV will interact in such a way that will produce a recombined exFeLV that can cause cancer. What has been less studied, however, is the function of enFeLV in the absence of exFeLV, and how enFeLV expression changes after exFeLV infection. This proposal aims to use next generation sequencing technology to examine how enFeLV responds to exFeLV infection in blood cells isolated from cats. Previous experiments have provided evidence that these enFeLV viral sequences may protect cats infected with exFeLV. As a result, the principal investigator believes that enFeLV activity may be stimulated following experimental infection with exFeLV. Outcomes of this work may help develop screening tests for domestic cat susceptibility to FeLV infection.