PRC II is an avian sarcoma virus encoding a gag-related protein, P105, which is unrelated to the RSV src gene product. However, both viruses appear to transform cells by aberrant phosphorylation of tyrosine residues on cellular proteins. The structural relationship between the apparent transforming proteins of PRC II, Fujinami sarcoma virus and feline sarcoma virus will be investigated, both at the level of nucleic acids and proteins. Recombinant DNA clones will be prepared containing PRC II proviral DNA and used to study in more detail the expression of this virus and its relationship to other retroviruses.