N-Mal-N-bis(PEG2-NHS ester) is a branched PEG derivative with a terminal maleimide group and two terminal NHS esters. The maleimide group will react with a thiol group to form a covalent bond, enabling the connection of biomolecule with a thiol. The NHS esters can be used to label the primary amines (-NH2) of proteins, amine-modified oligonucleotides, and other amine-containing molecules.