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Hydroxy-PEG4-acid sodium salt is a PEG linker containing a hydroxyl group with a terminal carboxylic acid (as sodium salt form). The free acid form is not stable due to the reaction of OH with PEG-COOH group to form polymer. The sodium salt form is stable for storage and shipping. The terminal carboxylic acid can react with primary amine groups in the presence of activators (e.g. EDC, or HATU) to form a stable amide bond. The hydrophilic PEG spacer increases solubility in aqueous media. The hydroxyl group enables further derivatization or replacement with other reactive functional groups.