Breast cancer is the second most common type of cancer and the leading cause of cancer death in women worldwide. Adjuvant tamoxifen therapy significantly slows down estrogen receptor (ER)-positive breast cancer progression, prolongs disease-free survival, and contributes to the decrease in breast cancer mortality (Musgrove and Sutherland, 2009). However, majority of the death from breast cancer is due to metastasis that is resistant to therapy (Musgrove and Sutherland, 2009). The frequent distant metastasis after therapy implicates that resistance to anti-cancer drugs might be the intrinsic characteristic of metastatic cancer cells. Tamoxifen-resistant breast cancer cells frequently exhibit mesenchymal-like morphology (Hiscox et al., 2006; Kim et al., 2009; Ward et al., 2012; Zhou et al., 2012), enhanced motility (McBryan et al., 2012; Zhou et al., 2012), and epithelial–mesenchymal transition (EMT) gene expression pattern (Iseri et al., 2011). Re-expression of microRNA-375 reverses both tamoxifen resistance and accompanying EMT-like properties in breast cancer (Ward et al., 2012). This promotes us to investigate whether EMT contributes to the acquisition of breast cancer tamoxifen resistance.