MC-Val-Cit-PAB-PNP is a cleavable ADC linker which contains a maleimide, a valine-citrulline (Val-Cit dipeptide, and a para-nitrophenyl (PNP) group. Maleimide is a thiol-specific covalent linker which is used to label cysteine residues in proteins while the PNP carbonate acts as a highly activated leaving group which may be used to react with amines. Val-Cit linkers are cleaved by Cathepsin B.