Fmoc-aminooxy-PEG4-acid is a PEG derivative with aminooxy group protected by Fmoc. The terminal carboxylic acid can be reacted with primary amine groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond. The hydrophilic PEG spacer increases solubility in aqueous media. The Fmoc group can be deprotected under basic condition to obtain the free amine which can be used for further conjugations.