Silencing REDD1 blocks bortezomib + MLN4924-induced cytotoxicity. MM.1R was transfected with nontarget siRNA or REDD1 siRNA. Twenty-four hours after transfection, cells were treated with 2 nM bortezomib, 2.5 µM MLN4924 or the combination. Growth inhibition was assessed using MTT assay (A); western blotting was used to check specific signaling proteins (B).