Fmoc-PEG6-Val-Cit-PAB-OH is a cleavable ADC linker featuring a Boc-protected amine, a hydrophilic PEG spacer, and a Val-Cit-PAB dipeptide. The benzyl alcohol on PAB can be utilized to attach reactive groups (e.g., PNP) for drug payload conjugation. The Fmoc protecting group is removable with piperidine, revealing a primary amine for amide bond formation in conjugation reactions. The Val-Cit-PAB segment is cleaved by cellular proteases, enabling efficient payload release into cells.