HyNic-PEG4-alkyne, a cleavable 4 unit PEG ADC linker, plays a critical role in the synthesis of antibody-drug conjugates (ADCs) [1]. This compound is also a click chemistry reagent featuring an Alkyne group, which enables it to participate in copper-catalyzed azide-alkyne cycloaddition (CuAAc) with molecules that have Azide groups.