Methyltetrazine-PEG4-hydrazone-DBCO, a cleavable 4-unit PEG ADC linker, serves as a click chemistry reagent in the synthesis of antibody-drug conjugates (ADCs) [1]. This compound features a DBCO group that facilitates strain-promoted alkyne-azide cycloaddition (SPAAC) with azide-bearing molecules. It also includes a Tetrazine group capable of engaging in an inverse electron demand Diels-Alder reaction (iEDDA) with TCO-bearing molecules.