The far-red fluorescent, lipophilic carbocyanine DiD is a longer-wavelength DiI analog. It is supplied as a solid form to facilitate direct application of crystals to membranes and 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.