PEG Acid is a class of PEG linker with one side of PEG containing a carboxylic acid (CO2H) group. PEG Acid will react with an amine-containing moiety in the presence of coupling reagents such as EDC, HATU. PEG Acid can also be converted to a N-hydroxysuccinimide active ester which is widely used in bioconjugation.