DBCO-PEG4-Maleimide is PEG linker containing a maleimide group and a DBCO moiety. The hydrophilic PEG spacer arm improves solubility in aqueous buffers. Maleimide group specifically and efficiently reacts with thiols to form stable thioether bonds. The low mass weight will add minimal spacer to modified molecules and will enable simple and efficient incorporation of DBCO moiety onto cysteine-containing peptides or other thiol-containing biomolecules. DBCO is commonly used for copper-free Click Chemistry reactions.