NH-Bis(PEG3-acid) HCl salt is a branched PEG derivative with a central amino group and two terminal carboxylic acids. The amino group is reactive with carboxylic acids, activated NHS ester, carbonyls, etc. The terminal carboxylic acids can be reacted with primary amino groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond.