We are studying two separate aspects of the biology of avian retroviruses: the mechanism of induction of avian osteopetrosis and the stability of proviral DNA. Avian retroviruses lacking oncogenes often produce primarily one form of non-acute disease. We have termed this disease preference "targeting." We are interested in defining which sequences in the viral genome are responsible for osteopetrosis "targeting" as a start to understanding the mechanism of disease induction. Our approach has been to construct a molecular clone of a virus which "targets" toward osteopetrosis (NTRE-2) and then to construct recombinant viruses by exchanging homologous restriction endonulease frangments between NTRE-2 and a virus which "targets" toward the production of lymphoma (RAV-60NY203) or which does not produce disease (RAV-O). Results of these studies suggest that sequences within the LTR are necessary for disease induction but that sequences within the viral structural genes most probably in gag or pol are required for the production of osteopetrosis. This represents the first association of a viral structural protein with the production of disease. We will extend these observations to smaller restriction fragments and ultimately to the level of DNA sequence analysis and site specific mutagenesis to define the sequences responsible for osteopetrosis induction. The similarity in structure between retrovirus proviral DNA and transposable genetic elements has been frequently commented upon. There is however little if any eveidence that proviruses possess any of the functional characteristics of transposable genetic elements. We have observed "instability" in the pattern of proviral DNA in several clones of td-RSV infected QT-6 cells. The patterns of "instability" observed include both loss and gain of proviral sequences and propagation of deletions into adjacent cellular DNA. We intend to use the techniques ol molecular cloning, DNA sequencing and chromatin structure analysis to determine whether specific sequences or DNA conformations can be correlated proviral instability.