Used in antibody-drug conjugates (ADCs) as a linker to connect cytotoxic drugs to monoclonal antibodies. The PEG3 spacer enhances solubility and flexibility, improving conjugation efficiency and stability in circulation. Val-Cil-PAB is a cleavable dipeptide linker that is specifically degraded by cathepsin B, an enzyme overexpressed in lysosomes of tumor cells, enabling targeted drug release inside cancer cells. The azide group allows for bioorthogonal conjugation via click chemistry, facilitating modular and efficient assembly of complex ADCs. This linker system is widely applied in the development of targeted cancer therapies to improve therapeutic index by minimizing off-target toxicity.