TGFB1 and glioblastoma: GBM is the most aggressive form of malignant glioma, while the occurrence of brain metastases is also suggested to impair cell-mediated immunity owing to increased transforming growth factor beta (TGF-β) levels, elevated numbers of circulating and tumor-infiltrating regulatory T cells (Treg) and non-productive inflammatory processes [3, 28, 51].