STAT3 and neoplasm: Signal transducer and activator of transcription 3 (STAT3) is abnormally activated in tumor cells and astrocytes of BCBM, and promotes the synthesis and secretion of downstream cancer-promoting signal molecules via intercellular signaling, thus inducing the M2 polarization of tumor-associated macrophages (TAM), inhibiting the infiltration of CD8+ T cells, and forming the immunosuppressive metastatic microenvironment [23,24].