5-Ethynyl-2'-deoxyuridine (EdU) is a nucleoside analog where the methyl group at the 5-position of thymidine is replaced by an ethynyl group. This structural modification makes it a powerful tool in life science research, particularly as a superior alternative to bromodeoxyuridine (BrdU) for detecting DNA synthesis in proliferating cells. The primary application of EdU is as a cell proliferation marker. Its terminal alkyne group allows for highly efficient, rapid, and specific detection via a copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC or "click chemistry") reaction with a fluorescent azide dye. This method is more sensitive, faster, and requires less harsh DNA denaturation steps compared to traditional BrdU detection using antibodies. It is widely used in cancer research, stem cell studies, and developmental biology. Beyond its role as a proliferation marker, the alkyne handle of EdU also makes it a valuable building block in chemical biology. It can be incorporated into nucleic acids and subsequently functionalized with various azide-tagged probes, enabling applications in bioorthogonal labeling, imaging, and the development of novel probes. Its utility extends to fields like targeted protein degradation, where it can serve as a chemical linker component due to its bioorthogonal reactivity.