MY-1B is a covalent inhibitor of the RNA Methyltransferase NSUN2 ( IC 50 : 1.3 μM). MY-1B stereoselectively ligands active-site cysteine residues (C271) of NSUN2. MY-1B can stereoselectively and covalently bind to PSME1 , disrupting the proteasome regulatory complex and downregulating the presentation of specific MHC-I subtypes.