These studies are designed to analyze the effects of oncogene expression on cell growth and differentiation. The functions of the proto-oncogenes c-fos and c-myc will be studied by observing the phenotype of both benign (immortalized) and malignant cells when oncogene expression is inhibited. REcombinant DNA vectors designed to produce "antisense RNA" have been introduced into mouse fibroblasts (3T3) and human myelomonocytic leukemia cells (HL60). These vectors contain a steroid inducible (MMTV) promoter which should theoretically allow regulated production of the "antisense RNA". Preliminary findings indicate that 10 fold fewer stable transformants are obtained when 3T3 cells are transfected with "antisense RNA" vector DNA in the presence of steroids, than in its absence or with appropriate vector controls. Inducible "antisense RNA" clones (stable transformants) will be identified and analyzed in two well-studied models of oncogene expression: 1) the of serum-deprived 3T3 cells to growth factors; and 2) phorbol ester induced differentiation of HL60 cells. Comparison of the biological response of clones with selective inhibition of c-fos or c-myc expression with the wild-type cells and with appropriate controls may further elucidate the role of these proto-oncogenes in cellular growth and differentiation.