Cytotoxic chemotherapy targets stromal cells in tissues from breast cancer patients. Tissue slices from 17 newly diagnosed breast cancer patients were treated with or without paclitaxel for 72 hours, then fixed in formalin and analyzed for proliferation and cell death using KI67 and TUNEL immunohistochemistry, respectively. (a) Representative examples of KI67 (left panel) and TUNEL (right panel) stained sections from tumor tissue slices from one patient incubated with or without paclitaxel. Scale bar, 50 μm. (b) The percentage of KI67 (left histograms) and TUNEL (right histograms) positive cells of both the tumor (upper panel) and the stromal compartment (middle panel) was assessed using an ×40 objective. Three to 7 randomly selected fields were examined for each section. The correlation of tumor and stromal response to paclitaxel regarding changes in proliferation and cell death is shown in the lower panels of the figure. Response to paclitaxel was defined as a reduction of KI67 positive cells by more than 10% (r2 = 0.33; slope = 0.26 ± 0.09; P = 0.015) or induction of TUNEL positive cells by more than 20% (r2 = 0.68; slope = 0.76 ± 0.15; P < 0.001).