We propose to continue the isolation and characterization of conditional and non-conditional mutants of avian oncoviruses. Emphasis will be placed on new deletion mutants and on temperature sensitive mutants in gag, pol or env. The possibility that nonexpressed endogenous viral genomes in chicken cells can recombine with temperature sensitive mutants to form wild type virus will be investigated. We will study recombination between avian oncoviruses. The hypothesis that heterozygotes are not essential precursors of recombinants will be investigated. Techniques to reduce the variability of recombination frequencies will be developed, and the effect of genetic relatedness on recombination rates and on the stability of recombinants will be studied. The effect of helper virus on the oncogenic spectrum of avian carcinoma virus MH2 and avian myelocytoma virus MC29 will be tested with different pseudotypes made by infecting MH2 and MC29 nonproducers with cloned helper viruses. Integration of multiple copies of avian oncoviruses in the same cell will be analyzed with specific fluorescent antibody tests for viral glycoprotein.