Acid-PEG4-Val-Cit-PAB-MMAE is a cleavable ADC linker featuring a carboxylic acid, a hydrophilic PEG spacer, and a Val-Cit-PAB dipeptide ligated to an MMAE payload. Carboxylic acids are reactive towards amines or alcohols 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.