Used in antibody-drug conjugate (ADC) development as a linker-payload component. The Fmoc-PEG4 segment provides solubility and acts as a spacer, while the Ala-Ala-Asn tripeptide is a substrate for cathepsin B, an enzyme overexpressed in tumor cells. Upon internalization and enzymatic cleavage, the Pab (para-aminobenzyloxycarbonyl) group facilitates release of the active drug, enabling targeted chemotherapy with reduced off-target effects. Commonly employed in prodrug strategies and site-specific conjugation for cancer therapeutics.