The long-term objective is to obtain a genetic map of an RNA tumor virus: Rous sarcoma virus. Towards this goal, we propose to study the genetics and biochemistry of Rous sarcoma virus replication. This will be achieved by: (a) the isolation of 25-50 temperature-sensitive replication mutants of Rous sarcoma virus; (b) the use of these mutants to determine the roles of viral gene products in initiation of infection, transcription of viral RNA, synthesis and maturation of viral proteins, and assembly of virions; (c) genetic mapping of replication mutations in relationship to each other and to known genetic markers for transformation, host range, and reverse transcriptase. BIBLIOGRAPHIC REFERENCES: Friis, R.R., Mason, W.S., Chen, Y.C. and Halpern, M. (1975). A replication defective mutant of Rous sarcoma virus which fails to make a functional reverse transcriptase. Virology, 64, 49-62. Halpern, M.S., Bolognesi, D.P., Friis, R.R. and Mason, W.S. (1975). Expression of the major viral glycoprotein of avian tumor virus in cells of CHF plus chick embryos. J. Virol., 15, 1131-1140.