The Hoechst stains are a family of fluorescent stains for labeling DNA in fluorescence microscopy. The blue fluorescent Hoechst dye is a cell permeable nucleic acid stain that has multiple applications, including sensitive detection of DNA in the presence of RNA in agarose gels, automated DNA determination, sensitive determination of cell number and chromosome sorting. Useful vital stain for the flow cytometric recognition of DNA damage and other viability measurements by monitoring the emission spectral shifts of the dyes. Because this fluorescent stain labels DNA, it can also be used to visualize nuclei and mitochondria. Hoechst 34580 can be excited by violet laser at 405 nm, and emit blue/cyan fluorescence light around an emission maximum at ~440 nm. It is cell-permeable and an excellent tool for flow cytometry studies.