Used as a linker in antibody-drug conjugates (ADCs), enabling targeted delivery of cytotoxic agents to cancer cells. The dipeptide Val-Cit is cleavable by cathepsin B, an enzyme overexpressed in tumor tissues, allowing selective release of the drug inside cancer cells. The PAB (para-aminobenzyloxycarbonyl) spacer facilitates efficient drug release upon cleavage, while the Alloc (allyloxycarbonyl) group serves as a protecting group during synthesis, which can be selectively removed under mild conditions. This linker system enhances the stability of ADCs in circulation and improves therapeutic index by minimizing off-target toxicity.