Hoechst 33342 azide is an excellent building block for developing site-specific nucleic acid probes via the well-known click chemistry, i.e., Copper-Catalyzed Azide–Alkyne Cycloaddition (CuAAC). It can be readily conjugated to alkyne-containing biomolecules (such as DBCO-modified antibodies, alkynyl-modified peptides and alkynyl-modified oligos). Hoechst 33342 is a DNA-specific dye that binds to the minor groove of double-stranded DNA. It emits blue fluorescence when bound to DNA. Its excitation maximum is around 350 nm, and the emission maximum is approximately 460 nm. Its ability to readily penetrate live cell membranes makes it suitable for staining live cells. This property allows researchers to monitor nuclear morphology and DNA content in living cells. Hoechst 33342 has also been used in the identification and isolation of stem cells based on their ability to efflux the dye. This property is exploited in a technique called side population analysis, commonly used in stem cell research.