DiI is a lipophilic membrane stain that diffuses laterally throughout the cell. Before being incorporated into cell membranes, it fluoresces weakly. This orange-red fluorescent dye has a spectrum similar to tetramethylrhodamine and is commonly used as a long-term tracer of neurons and other cells. DiI is also available as a solid (D-282), paste (N-22880), or crystal (D-3911).