STAT3 and breast cancer: Since five of the top twenty differentially expressed phosphoprotein targets were components of the JAK/STAT pathway (STAT1, STAT3, TYK2, JAK1, JAK2) and STAT1 was the most differentially expressed total protein, we reasoned that STAT signaling might be a mediator of endocrine sensitivity and tamoxifen/fulvestrant resistance in breast cancer, prompting us to explore this association further.