N-(Amino-PEG4)-N-Biotin-PEG4-acid is a PEG derivative with biotin, terminal amine and carboxylic acid moieties. The biotin group is used for the conjugation of proteins and can also be used for labeling. The amine group can react with carboxylic acids, activated NHS esters and other carbonyl groups. The carboxylic acid can react with primary amines in the presence of activators such as EDC and DCC to form stable amide bonds.