STAT5A and breast cancer: We have also shown that acutely blocking STAT3 and STAT5 phosphorylation using SH-4-54, a novel, structurally unique small molecule inhibitor that effectively targets interactions with the pTyr-SH2 domain to block both STAT3 and 5 dimerization and subsequent DNA-binding, also providing a promising means of targeting brain cancer stem cells [10], significantly up-regulates xCT expression and system xc- activity in several human breast cancer cell lines [25].