Abstract Growth factors are extracellular polypeptide mediators of intercellular signalling. They exert their action by binding to specific receptors expressed by target cells, thereby initiating signal transduction processes inside the cell. Depending upon the nature of the target cell and the identity of the specific signalling mechanisms involved, changes in cell behaviour ensue. These can include the induction of cell differentiation, migration, death or survival, as well as activation or inhibition of cell multiplication (reviewed in ref. 1). The significance of growth factors in the context of on cogenesis is therefore that they have the property of evoking precisely those cellular responses that are subverted in the course of oncogenic transformation and tumorigenesis. Abnormal expression of growth factors, and consequential activation of cell signalling is therefore a potentially significant mechanism of oncogenic transformation.