The primary goal of this research is to use EIAV as a natural biological model of persistent infections to examine the molecular mechanisms of viral gene expression which correlate with latency and disease in the infected host. There are three specific aims. First, the investigator proposes to characterize the temporal patterns of provirus integration and viral transcription during persistent and cytopathic infections in vitro. The second is to try to characterize the patterns of provirus integration and viral gene expression during stages of latency and disease in experimentally infected ponies. The third is to attempt to utilize infectious molecular clones of EIAV to identify and characterize viral determinants important in regulating virus replication, virulence and host pathogenesis. In the first two objectives, DNA and RNA polymerase chain reaction (PCR) will be used to define the patterns of provirus integration and viral transcription in persistently- and cytopathically-infected equine cells and from tissues of experimentally infected ponies during stages of acute, chronic and inapparent disease. The third objective will use infectious molecular clones of EIAV in mutagenesis studies to identify viral genes and cis-regulatory sequences essential for virus replication and virulence in horses.