Gly-Gly-Gly-PEG3-TCO is a cleavable 3 unit PEG ADC linker used in the synthesis of antibody-drug conjugates (ADCs)[1]. Gly-Gly-Gly-PEG3-TCO is a click chemistry reagent, it contains a TCO group that can undergo an inverse electron demand Diels-Alder reaction (iEDDA) with molecules containing Tetrazine groups.