(1) S100A9 is up-regulated in GBM tissues and is associated with poor prognosis, and can be used as an independent prognostic factor in GBM patients; (2) S100A9 can be used as an effective tool to predict the immunotherapy response for GBM patients; (3) S100A9 can promote the malignant progression of GBM; (4) S100A9 is mainly expressed in monocytes and macrophages, and the modest amount is also expressed in malignant tumor cells; (5) S100A9 contributes to the migration of M2 macrophages. Here, S100A9 is linked to cancer.