Barry Sharpless, at the Scripps Research Institute, proposes in 2001 the concept of "click chemistry": a design philosophy for chemical reactions that should be simple, extremely efficient, generate very few unwanted byproducts, and work reliably under mild conditions, similar to the mechanical simplicity of a fastener that snaps into place with a single motion ("click"). Morten Meldal, at the Technical University of Denmark, independently and almost simultaneously discovers the specific reaction that would become the most widely used example of click chemistry: the copper-catalyzed azide-alkyne cycloaddition, an extraordinarily reliable and selective reaction between two chemical groups that barely react with any other molecule present in a complex biological system. Carolyn Bertozzi, at Stanford University, extends the concept to living systems by developing click chemistry reactions that work inside living organisms without interfering with any of the thousands of natural chemical processes of the cell — what she calls "bioorthogonal" chemistry — allowing specific molecules, such as particular sugars or proteins, to be labeled and tracked directly inside living cells or even whole organisms without disturbing their normal function. These techniques are today widely used in biomedical research, targeted drug design, and the development of precision oncology therapies.