EdU (5-Ethynyl-2′-deoxyuridine), a thymidine analogue, which can be incorporated into cellular DNA and the subsequent reaction of EdU with a fluorescent azide in a copper-catalyzed [3+2] cycloaddition (“Click” reaction) are developed as a method for detection of DNA synthesis. 5-Ethynyl-2′-deoxyuridine is an alkyl chain-based PROTAC linker that is applicable to the synthesis of PROTACs.