DAPI is a fluorescent stain that binds strongly to DNA. It is commonly used in biological research to visualize cell nuclei under a fluorescence microscope. When exposed to ultraviolet light, DAPI emits a blue fluorescence, allowing scientists to easily identify and count cells in various samples. This dye is particularly useful in studies involving cell division, apoptosis, and cellular localization of genetic material. By highlighting the nuclei, researchers can assess the health and status of cells, making DAPI an essential tool in fields like cell biology and genetics.