Used as a linker in antibody-drug conjugates (ADCs), particularly in the construction of prodrugs for targeted cancer therapy. The Fmoc group acts as a protective moiety that can be selectively removed under mild basic conditions, allowing controlled activation of the conjugate. The Ala-Ala dipeptide serves as a substrate for specific proteolytic enzymes, such as cathepsin B, which are overexpressed in tumor environments. Upon internalization of the ADC into cancer cells and subsequent lysosomal degradation, the dipeptide is cleaved, releasing the PAB (para-aminobenzyloxycarbonyl) spacer that facilitates efficient liberation of the cytotoxic payload. This system enhances the specificity and reduces off-target toxicity of chemotherapeutic agents.