Acid-PEG2-SS-PEG2-acid is a cleavable PEG linker with carboxylic acid groups at both terminus. The disulfide bonds can be cleaved via a reduction reaction using Dithiothreitol (DTT) reagent. The terminal carboxylic acids can be reacted with primary amines in the presences of activators such as EDC and DCC to form stable amide bonds. The hydrophilic PEG linkers can increase the water solubility of the compound in aqueous media.