Used in antibody-drug conjugates (ADCs) as a cleavable linker to connect cytotoxic drugs to monoclonal antibodies. The Val-Cit-PAB segment is a substrate for cathepsin B, an enzyme overexpressed in tumor cells, enabling selective drug release inside cancer cells. Mal (maleimide) allows conjugation to thiol groups on antibodies, while PEG1 spacer improves solubility and stability. PNP (para-nitrophenyl) acts as a leaving group during conjugation with payloads like auristatins or may serve as a prodrug moiety. This linker system enhances the therapeutic index of ADCs by promoting targeted delivery and reducing systemic toxicity.