The far red fluorescent, lipophilic carbocyanine DiIC₁₈(5)-DS contains sulfonate groups to improve water solubility. It is weakly fluorescent in water but highly fluorescent and quite photostable when incorporated into membranes. The sulfonate groups incorporated into this DiI analog improves water solubility. 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.