A selective inhibitor of the β5i (LMP7) subunit of the immunoproteasome (IC50s = 65 and 73 nM for mouse and human, respectively) that demonstrates significantly weaker activity at the β5 subunit of the constitutive proteasome (IC50s = 0.92 and 1.04 µM for mouse and human, respectively); blocks the production of proinflammatory cytokines and inhibits TH1 and TH17 cell differentiation; attenuates disease progression in several experimental models of autoimmune disorders as well as in some hematologic malignancies.