CLA induced apoptosis of breast cancer epithelial cells is associated with estrogen receptor α (ERα) in co-culture. MCF-7 (A), MDA-MB-231 (B), ERα transfected MDA-MB-231, and MDA-MB-231-ERα (C), were cultured alone or co-cultured with normal human breast stromal cells (NSC), or co-cultured with cancerous human breast stromal cells (CASC). Cells were treated by t10, c12-CLA for 7 days and cell apoptosis of MCF-7 (A), MDA-MB-231 (B) or MDA-MB-231-ERα (C) was detected from each group separately by caspase-3/7 activity assay. (D) Bcl-2 protein expression of MCF-7, MDA-MB-231 or MDA-MB-231-ERα was determined by western blot analysis. CLA stands for 40 μM t10, c12-CLA. Bars represent mean ± SD, n = 3. *p < 0.05 stands for t10, c12-CLA group comparison of caspase-3/7 activity; +p < 0.05 for control versus t10, c12-CLA.