Fmoc-PEG4-Val-Cit-PAB-OH is an enzyme-cleavable ADC linker that incorporates a Boc-protected amine, a hydrophilic PEG spacer, and a Val-Cit-PAB dipeptide. The benzyl alcohol on PAB can be used to attach reactive groups (such as PNP) for drug payload conjugation. The Fmoc protecting group can be removed with piperidine to expose a primary amine, facilitating amide bond formation. Val-Cit-PAB is cleaved by cellular proteases to efficiently release the payload inside cells.