The major thrust of this study is to elucidate the molecular basis of neoplastic transformation of normal tissues. Two experimental model systems were used in this project: (1) Intracisternal A particle (IAP) containing neoplasms, a BALB/c mammary tumor cell line, its hybrid and cybrid clones, a plasma cell tumor (MOPC 104E), and feral mouse cell lines of both Mus musculus and Mus cervicolor; and (2) KiMSV (RHHV), a transforming rat virus isolated in this laboratory by co-cultivating Kirsten sarcoma virus transformed non-producer rat kidney cells (K-NRK) with a chemically induced rat hepatoma cell line (HTC-H1). Multidisciplinary approaches were required in this investigation. They involve nucleic acid and protein chemistry, tissue culture, cytogenetics, immunology and tumor biology. Since we have completed extensive characterizations morphologically, biologically, biochemically and immunologically of both these retraviruses, our current emphasis of research efforts focuses on: (1) integrations of horizontally transferred viral genome and its localization in the host cell; (2) regulation of viral gene expressions; (3) integration site(s) for exogenous viral DNA sequences; (4) mapping of a viral genome; (5) viral gene conservation during the evolution of the Mus genus.