DiI is a lipophilic membrane stain that diffuses laterally to stain the entire cell. It is weakly fluorescent until incorporated into membranes. This orange—red-fluorescent dye, which is spectrally similar to tetramethylrhodamine, is often used as a long-term tracer for neuronal and other cells. DiI is also available as a solid (D-282), as a paste (N-22880) or as crystals (D-3911).