N3-PEG3-CH2CH2COOH a PEG-based PROTAC linker can be used in the synthesis of BI-3663 (HY-111546), BI-4216 and BI-0319. Azido-PEG3-acid is also a non-cleavable 3 unit PEG ADC linker used in the synthesis of antibody-drug conjugates (ADCs). N3-PEG3-CH2CH2COOH is a click chemistry reagent, it contains an Azide group and can undergo copper-catalyzed azide-alkyne cycloaddition reaction (CuAAc) with molecules containing Alkyne groups. Strain-promoted alkyne-azide cycloaddition (SPAAC) can also occur with molecules containing DBCO or BCN groups.