TCO-PEG4-NHS ester is a PEG-based PROTAC linker can be used in the synthesis of PROTACs. TCO-PEG4-NHS ester is a cleavable ADC linker used in the synthesis of antibody-drug conjugates (ADCs)[1]. TCO-PEG4-NHS ester 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.