FAST DiI is an unsaturated analog of DiI (DiIC₁₈(3)) that reportedly migrates ∼50% faster. It is 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.