Cyclooctyne-O-amido-PEG4-PFP ester, a non-cleavable 4-unit PEG ADC linker, is employed in the synthesis of antibody-drug conjugates (ADCs) [1]. This click chemistry reagent features an Alkyne group and can participate in copper-catalyzed azide-alkyne cycloaddition (CuAAc) with molecules that have Azide groups.