The complexity of growth factors and growth factor receptors that are aberrantly expressed, as well as the mutational events that either directly cause or influence the expression of these and other gene products, should provide in the near future multiple diagnostic, prognostic indicators or targets for therapeutic intervention. It seems reasonable to expect that soon the search for aberrantly expressed gene products in breast cancer cells will merge with the search and characterization of somatic mutations that are selected during tumour progression. Clearly, the current rapid development of new molecular biological methodologies aimed at detecting and cloning of RNA sequences that are aberrantly expressed in breast tumour cells, as well as molecular probes and reagents to detect and physically map mutated genes on affected chromosomes, should accelerate the effort to identify targets for therapeutic intervention. We are at the beginning of this learning curve, but already several potential target gene products have been identified. A major challenge will be to sort out those approaches and reagents that appear efficacious on the basis of results from in vitro and in vivo model systems that will actually have an impact on the treatment of the disease in the clinic. Reagents that target some of these gene products are currently in clinical trials; however, there are others such as immunotherapy against the mutated TP53 protein and human CG treatment of high risk breast cancer patients that warrant testing in this context.