Biotin-PEG-Mal, MW 10K is a PEG derivative containing a biotin group and a maleimide group. The biotin group is used for the conjugation of proteins and can also be used for labeling. The maleimide can react with thiol groups from pH 6.5 to 7.5. The hydrophilic PEG chain increases the water solubility properties of the compound.