TCO-PEG1-Val-Cit-PABC-OH is a cleavable 1 unit PEG ADC linker used in the synthesis of antibody-drug conjugates (ADCs). TCO-PEG1-Val-Cit-PABC-OH 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.