N-(acid-PEG3)-N-bis(PEG3-biotin) is a branched PEG linker with biotin and carboxylic acid moieties. The biotin group is used for the conjugation of proteins and can also be used for labeling. The carboxylic acid group can react with primary amines in the presence of activators such as EDC and DCC to form stable amide bonds. The hydrophilic PEG linker increases the water solubility of the compound in aqueous media.