The replication of the endogenous avian retrovirus in non-permissive chicken cells will be examined. Biological studies will examine the nature of the restriction in different genetic inbred lines, the mechanism by which the endogenous virus persists in non-permissive cells and the type of genetic event that leads to emergence of the variant. In order to characterize the restricted event, the biological activity of unintergrated intermediates will be examined. Further, the integration of the endogenous viurs in ramdom and clonal populations of non-permissive cells will be characterized using restriction endonuclease and Southern DNA transfer analysis. Both the endogenous virus and the variant to which it gives rise will be cloned by recombinant DNA techniques. Biochemical analysis will identify those sequences characteristic of the variant that function to provide replication in non-permissive cells.