Replication of Rous Sarcoma Virus (RSV) will be studied in cells whose DNA synthesis is stopped by either natural differentiation mechanism or by deprivation of serum: 1. Replication of RSV ts 24 (Prague strain) in differentiating chicken myogenic cell culture. Myogenic cells will be infected with RSV ts 24 (class T mutant having temperature sensitive transformation gene product) and will be kept at new permissive temperature. The myogenic cells will develop into muscles and the host DNA synthesis stops completely. We shall study the state of viral genome in these muscle cells. We shall confirm our preliminary conclusion that muscle cells do not produce virus and find out whether transcription, or translocation step is blocked. We shall also confirm our preliminary observation that RSV does not influence the well differentiated myotubes and study the nature of viral DNA in these cells. We shall find out why replicating myogenic cell is susceptible to RSV while well developed myotubes (whose DNA synthesis is stopped) are not. 2. Studies on the requirement of host DNA synthesis for establishing viral genome starting from the classical observation of Temin, that host DNA synthesis is required for establishing viral genome, we shall study the mechanism of integration of viral genome into host chromosomes. We shall test the possibility that covalently closed circular viral DNA may be found in the cells when host DNA synthesis was stopped at the time of infection. We shall resume the host DNA synthesis and observe the kinetics of viral DNA integration. Finally we plan to develop all free nuclear system which would integrate viral genome into the duo moromes and study the biochemical mechanism of viral DNA integration.