STAT3 and breast cancer: The biological mechanism of brain metastasis in breast cancer patients is complex and may be caused by the dysregulation of multiple cellular signaling pathways, such as PI3K, AKT, JAK-STAT3, MAPK-ERK1, NF-κB, Wnt–β-catenin STAT3, p53, TGF-β, EGF, NF-Kβ, and others [32,38].