Maleimide PEG Amine (Mal-PEG-NH2) is a linear heterobifunctional PEG reagent containing maleimide and amine, commonly used as a crosslinker or spacer between two different chemical entities. Maleimides can react with thiols, SH, sulfhydryl or sulfhydryl groups, and amines can react with carboxylic acids or activated NHS esters. The PEG moiety in the heterofunctional PEG derivatives provides water solubility, biocompatibility and flexibility. Applications are particularly applicable to antibody drug conjugates.