Amino-PEG4-Val-Cit-PAB-MMAE is a cleavable ADC linker featuring a primary amine, a hydrophilic PEG spacer, and a Val-Cit-PAB dipeptide ligated to an MMAE payload. Primary amines readily form amides with carboxylic acids in the presence of activator reagents such as HATU or EDC, while the Val-Cit-PAB motif is an enzyme-cleavable linker. This linker efficiently releases the toxin, MMAE, to target cells to cause cellular apoptosis.