The erbB-2 receptor plays an important role in the prognosis of breast cancer and is expressed at high levels in nearly 30% of tumors in breast cancer patients. While evidence accumulates to support the relationship between erbB-2 overexpression and poor overall survival in human breast cancer, understanding of the biological consequence(s) of erbB-2 overexpression remains elusive. The discovery of heregulin has allowed us to identify a number of related but distinct biological endpoints which appear responsive to signal transduction through the erbB-2/4 receptor. These endpoints of growth, invasiveness, and differentiation have clear implications for the emergence, maintenance, and/or control of malignancy, and represent established endpoints in the assessment of malignant progression in human breast cancer. Preliminary studies in vitro have shown that heregulin induces a biphasic growth effect on cells with erbB-2 overexpression. Interestingly, we observed that expression of heregulin correlates with a more aggressive/invasive, vimentin-positive phenotype in breast cancer cells lines. Therefore, we have postulated that heregulin is involved in breast cancer tumor progression. We have shown that heregulin induces in vitro chemoinvasion and chemotaxis of breast cancer cells as well as growth in an anchorage dependent and independent manner. Interestingly, a heregulin neutralizing antibody inhibits chemotaxis and results in cell growth inhibition and blockade of the invasive phenotype. Strikingly, genetically engineered cells which constitutively express heregulin demonstrate critical phenotypic changes that are associated with a more aggressive phenotype. Specifically, these cells are no longer dependent on estrogen for growth and are resistant to tamoxifen in vitro and in vivo, and moreover these cells metastasize to lymph nodes in athymic nude mice. These tumors appear to have lost bcl-2 expression as compared with the control tumors. In addition, presumably by activation/regulation of topoisomerase II, the heregulin-transfected cells become exquisitely sensitive to doxorubicin and VP-16. Clearly, mechanistic aspects of the erbB-2/4 and heregulin interaction need to be understood from a therapeutic standpoint which could provide additional insights into synergistic treatments for certain patients, or improve treatment regimens for a large number of women. The study of heregulin and its co-expression with erbB-2/4 receptor and the assessment of its involvement in the progression from the in situ stage of breast tumors to the invasive one will additionally increase the relevance of heregulin as a prognostic/diagnostic factor. We believe that our studies provide new insights into breast cancer diagnosis, prognosis, and treatment.