The long term goals of this project are to determine the mechanism of recombination of virus DNA with the host chromosome and to determine how intracellular virus DNA becomes infective (or matures). We have isolated and studied mutants of coliphage lambda that are unable to perform these processes. An analysis of phage and host mutants defective in virus-host recombination has yielded an estimate of the number and sizes of the proteins required and has given information about how the proteins interact with the DNA. An analysis of mutants defective in virus DNA maturation has suggested a model for this process. The properties of both of these types of mutants have enabled us to develop a new vector for the cloning of fragments of eukaryotic DNA. BIBLIOGRAPHIC REFERENCES: Enquist, L. and Weisberg, R.: The Red Plaque Test: A Rapid Method for Identification of Excision Defective Variants of Bacteriophage Lambda. Virology 72: 147-153, 1976. Leder, P., Tiemeier, D. and Enquist, L.: EK2 Derivatives of Bacteriophage Lambda Useful in the Cloning of DNA from Higher Organisms: The lambda gtWES System. Science 196: 175-177, 1977.