STAT5A and breast cancer: Like in liver cancer, STAT5 also seems to counteract the oncogenic effects of STAT3 as part of the tumor-suppressive role of STAT5 in breast cancer, as suggested by a report that STAT5 and STAT3 activation have opposing effects, and that co-activation of STAT5 and STAT3 in breast cancer cells decreases cell proliferation and sensitizes cells to chemotherapeutic drugs paclitaxel and vinorelbine, in contrast with STAT3 activation alone [24].