Hydroxyl PEG Amine (HO-PEG-NH2) is a heterobifunctional PEG, and the heterobifunctional hydroxy PEG amine product is commonly used as a crosslinker or as a spacer between two different chemical entities. The amine group reacts with the activated NHS ester to form a stable amide bond. Hydroxyl groups can be further derivatized. The PEG moiety in the heterofunctional PEG derivatives provides water solubility, biocompatibility and flexibility. Applications are particularly suitable for the development of antibody drug conjugates (ADCs).