STAT3 and cancer: Taken together, the interleukin-6 (IL-6) and the signal transducer and activator of transcription 3 (STAT3) or IL-6/STAT3 pathway is a central regulator of breast cancer metastasis by promoting breast cancer cell proliferation, the development of metastasis (by the epithelial–mesenchymal transition (EMT) and enriching cancer stem cells (CSCs)), and suppressing apoptosis.