Endogenous proviruses and other retroviral-like sequence have been demonstrated to represent a significant portion of the genome of many eukaryotes. The majority of these retro-elements can no longer replicate to produce new infectious viral particles, but may nevertheless still have profound effects on their host organism. They are often found in multiple copies scattered throughout the chromosomal complement. Although most are probably benign, many have been implicated in such phenomenon as mutagenesis, carcinogenesis, recombination and transposition. Although many of these genetic elements have been described and sequenced in a range of eukaryotes, all evidence indicates that there are far more that have yet to be studied. Also, even for those that have been categorized to some extent, there is little we know about how that sequence acts as a "population" within a species. For instance, given a endogenous provirus where a single element has been sequenced, and where evidence indicates that the same element exists in numerous copies in the genome, we can now ask questions concerning the variation (sequence), and distribution of these elements throughout the genome. The answers to such questions can supply insight into even more meaningful questions concerning the modes of transmission, and evolution of these sequences. It is the goal of this project to undertake a detailed analysis of a population of endogenous proviruses, the first members-of which were described by Stavenhagen and Robbins in 1988, and Zimmerer and Passmore in 1991. The specific aims of the project are to: 1) determine the patterns of distribution of these elements within and between species of Mus by Southern blot analysis, 2) determine the complete sequence of a more recently described third member of this population, using anchored PCR, cloning and subsequent sequencing, and 3) clone and characterize other members of this group by screening lambda DNA libraries with probes specific for this endogenous provirus.