USP15 and glioblastoma: TGF-β activity in GBM is modulated by several deubiquitinating enzymes (DUBs); first, it was described that TGF-β activity could be enhanced due to the amplification of a deubiquitinating enzyme, USP15, which binds to the SMAD7–SMURF2 complex and stabilizes type I TGF-β receptors, resulting in enhanced TGF-β signaling.