The orange-red fluorescent lipophilic carbocyanine Δ⁹-DiI is an unsaturated DiI analog that may migrante faster than DiI (DiIC₁₈(3)) within membranes. It is an oil at room temperature and weakly fluorescent in water but highly fluorescent and quite photostable when incorporated into membranes. It has an extremely high extinction coefficient and short excited-state lifetimes (~1 nanosecond) in lipid environments. Once applied to cells, the dye diffuses laterally within the plasma membrane.