GFAP and cancer: Furthermore, GFAP+ reactive astrocytes expressing phosphorylated STAT3 (pSTAT3+ GFAP+ cells) are essential for BrM colonization and outgrowth via Chi3L1, a STAT3 target gene expressed by stromal cells in the BrM microenvironment (96).Taken together, the unique interactions between cancer cells and brain-specific stroma cell types, along with the immunosuppressive microenvironment in brain metastases, play crucial roles in immunosuppression and tumor progression.