Additional studies involving exogeneous addition of an upstream activator (TGF-β1 protein) demonstrated significant sensitization of GBM cells to irinotecan and simvastatin, potentially indicating that TGF-β up-regulation may be “addicting” GBM to this pathway and sensitizing to TGF-β inhibition by irinotecan plus simvastatin. This evidence concerns the gene TGFB1 and glioblastoma.