A widely studied model for the mechanism of oncogenic transformation involves the insertion of an oncogene or the activation of a proto-oncogene in a normal cell. Both the nature of the oncogene- and proto-oncogene-products and the mechanisms by which they act to transform the cell are poorly understood. Oncogenic transformation in tissues is believed to involve a phenomenon called cooperativity in which more than one oncogene or proto-oncogene must be activated to effect transformation. It is proposed that the various oncogene and proto-oncogene-products that lead to the transformation of a given cell type either mimic or are the enzymes and other proteins involved in mediating the mitogenic signal to which that cell type responds. Cooperativity among oncogenes follows from this because increased activity or increased levels of mediators in the mitogenic sequence could increase the efficiency of transmission of mitogenic signals from the cell surface, thereby lowering the threshold level of growth factor activity required to initiate DNA synthesis.