<qa>  What is the prime example of a predictive marker?Predictive markers in breast cancer: Prognostic factors like tumor size and nodal involvement are important indicators for breast cancer survival but have not been shown to be predictive of sensitivity to chemotherapy. With endocrine therapy, the prime example of a predictive marker is estrogen receptor ER expression, which predicts for response to tamoxifen and other endocrine treatments. However, predictive markers for chemotherapy are not established. Overexpression of c-erbB-2 might be associated with decreased response to CMF and increased response to anthracycline-based treatment, but these observations are still contentious. Expression of topoisomerase II may also reflect responsiveness to anthracycline chemotherapy. Recently, some emerging data also suggest that c-erbB2 may be a marker of taxane sensitivity. As such, we lack predictive biomarkers that could give early information on how effective chemotherapy is and whether additional treatment might be beneficial. A test for chemotherapy sensitivity, equivalent to ER in predicting response to endocrine therapy, would greatly facilitate treatment decisions so that in an ideal scenario, the treatment of each individual patient could be based on specific features of her disease. 