An irreversible inhibitor of the chymotrypsin-like β5 subunit of the constitutive 20S proteasome (IC₅₀ = 5.2 nM) and the β5i subunit of the immunoproteasome 20Si (LMP7; IC₅₀ = 14 nM) with minimal cross reactivity to other proteases; induces cell cycle arrest and…