9-DiI methanesulfonate is widely used as a fluorescent dye in biological research, particularly for cell membrane labeling and tracking. Its lipophilic nature allows it to integrate into lipid bilayers, making it ideal for staining living cells, neurons, and other cellular structures. This dye is commonly applied in fluorescence microscopy and imaging studies to visualize cell dynamics, such as membrane fusion, cell migration, and intracellular transport. Additionally, it is utilized in neuronal tracing experiments to map neural pathways and study synaptic connectivity due to its ability to diffuse along cell membranes. Its stability and bright fluorescence make it a valuable tool in live-cell imaging and long-term tracking applications.