AbstractInhibiting estrogen receptor (ER) function with specific estrogen receptor modulators (SERM) can achieve a primary response in cancer patients; however, intrinsic or subsequently acquired resistance to the therapy remains a major obstacle in treatment. The pure anti‐estrogen fulvestrant has been shown to be a promising antagonist of ERα in treating advanced breast cancer. However, our knowledge of the mechanisms governing cellular responsiveness to this agent is limited. Here we show that down‐regulation of the nonreceptor tyrosine kinase c‐ABL enhanced sensitization to fulvestrant in breast cancer cells. Blocking c‐ABL kinase activity with the inhibitor imatinib further increased ERα downregulation induced by fulvestrant, decreased the number of proliferating cells entering the cell cycle, and increased cellular sensitivity to fulvestrant treatment. Conversely, introducing kinase‐activated c‐ABL can rescue fulvestrant‐induced ERα downregulation. Consistent with the effects of imatinib, the silencing of endogenous c‐ABL increased the sensitivity of breast cancer cells to fulvestrant treatment. These results demonstrate a role for c‐ABL in mediating resistance to the pure anti‐estrogen fulvestrant. © 2010 Wiley‐Liss, Inc.