Methyltetrazine-PEG4-hydrazone-DBCO is a cleavable 4 unit PEG ADC linker used in the synthesis of antibody-drug conjugates (ADCs). Methyltetrazine-PEG4-hydrazone-DBCO is a click chemistry reagent, it contains a DBCO group that can undergo strain-promoted alkyne-azide cycloaddition (SPAAC) with molecules containing Azide groups. Methyltetrazine-PEG4-hydrazone-DBCO also contains a Tetrazine group that can undergo an inverse electron demand Diels-Alder reaction (iEDDA) with molecules containing TCO groups.