Breast cancer is a heterogeneous disease that can be stratified based on the expression of molecular markers such estrogen receptor (ER), progesterone receptor and epidermal growth factor receptor 2.Breast tumor kinase (BRK) is a non-receptor tyrosinc kinase expressed in the majority of human breast tumors and breast cancer cell lines, but its expression has not been detected in normal mammary gland.The overexpression of BRK has been shown to sensitize mammary epithelial cells to mitogenic signaling and to promote cell proliferation and tumor formation.However, there are still several unanswered questions about the cellular and physiological roles of BRK and its clinical implications in breast cancers.I will discuss recent insights into the role of BRK in tumor formation as well as the potential clinical implications of BRK, especially in ER-positive breast cancers.