Biotin-PEG4-SS-azide, a biotin-labeled ADC linker, is primarily utilized in the synthesis of antibody-drug conjugates (ADCs). This compound serves as a click chemistry reagent featuring an Azide group, enabling it to partake in copper-catalyzed azide-alkyne cycloaddition reactions (CuAAc) with alkyne-bearing molecules. Additionally, it can engage in strain-promoted alkyne-azide cycloaddition (SPAAC) with molecules containing DBCO or BCN groups.