Molecular alterations in human breast cancer have provided both a better understanding of the disease process and an important rationale for clinical decision-making. Estrogen receptor (ER) and progesterone receptor (PR) were among the first alterations in any solid cancer wich proved to be clinically important. ER and PR have been shown to be correlated with clinical outcome and with responsiveness to hormonal therapies. Considerable evidence indicates that another receptor, HER-2/neu, is equally important in breast cancer specimens. HER-2/neu amplification and overexpression have been correlated with shorter disease-free interval and shorter overall survival in both node-positive and node-negative disease. Some investigators have not been able to confirm these clinical outcome correlations in their studies. They have addressed a number of potential problem areas with clinical correlation studies in an effort to resolve some controversy surroundings the clinical utility of HER-2/neu. Issues related to study design, especially sample size, sensitivity and specificity of analytic methods used for gene amplification / overexpression and the potential confounding effect of differential treatment responsiveness, have been considered. During the current funding period several studies were performed to address these issues. A wide range of antibody immunoreactivities were demonstrated among 28 different HER-2/neu antibodies in paraffin-embedded tissue specimens with known gene amplification and expression levels which were characterized in frozen specimens by Southern, Norther and Western blot analysis and immunohistochemical staining. The most sensitive antibodies were identified and used in additional studies. A method for quantitation of HER-2/neu protein using molecularly engineered cells and computerized image analysis was described and tested. Fluorescence in situ hybridization (FISH) was shown to be suitable for gene amplification studies in archival clinical specimens. Two retrospective studies of HER-2/neu, gene amplification or expression were conducted in women with axillary lymph-node negative breast cancer. Both studies were restricted to women who had no adjuvant radiation therapy, chemotherapy or hormone therapy and both studies demonstrated a significant correlation between HER-2/neu, alteration and poor clinical outcome. Similar correlations between poor survival and HER-2/neu amplification/overexpression were demonstrated in studies of patents with endometrial cancer and salivary gland carcinoma. Currently and in our proposal for the future renewal period, we are addressing issues related to HER-2/neu receptor as a predictor of responsiveness to treatment. In a preliminary, prospective study of taxol (paclitaxel) chemotherapy in women with recurrent or metastatic breast cancer we have observed a correlation between HER-2/neu overexpression and prolonged overall survival. Paradoxically, this suggests greater responsiveness to taxol chemotherapy among breast cancers which overexpress a marker or more aggressive disease. A prospective clinical trial of HER-2/neu as a predictor of responsiveness to chemotherapy is proposed. A retrospective study of HER-2/neu as a predictor of tamoxifen responsiveness is also planned. These studies should lead to a greater understanding of this molecular genetic alteration and its use in clinical management decisions.