MLN2238 is a reversible inhibitor of the chymotrypsin-like proteolytic β5 site of the 20S proteasome (IC50 = 3.4 nM; Ki = 0.93 nM in cell free assays). It inhibits the caspase-like β1 and trypsin-like β2 proteolytic sites with IC50 values of 0.03 and 3.5 µM, respectively. MLN2238 is the biologically active form of the prodrug MLN9708 (Item No. 18386), which has shown antitumor activity both in cell-based assays and in xenograft models and has been examined in phase III clinical trials for potential antimyeloma activity.