Boc-PEG4-Val-Cit-PAB-OH is an enzyme-cleavable ADC linker that features a Boc-protected amine, a hydrophilic PEG spacer, and a Val-Cit-PAB dipeptide. The benzyl alcohol on PAB is useful for attaching reactive groups (e.g., PNP) to bind with drug payloads. Acidic conditions can remove the Boc protecting group, exposing the primary amine for conjugation reactions to form amides. The Val-Cit-PAB segment can be cleaved by intracellular proteases, allowing efficient payload release within cells.