 In serum, TTR and RBP4 bind in a 1 to 1 stoichiometry (two molecules of TTR combine with two molecules of RBP4 to form a complex with a total molecular weight of approximately 80,000 Daltons).Retinol-binding protein 4 has been a drug target for eye diseases as RBP4 is the sole carrier for retinol, which is an essential nutrient for the visual cycle.Animal studies using RBP4-antagonists showed that lowering RBP4 can lead to reduction in the accumulation of lipofuscin that leads to vision loss in eye diseases like Stargardt's disease and macular degeneration.An animal study using ABCA4 knockout mouse proved that reduction in serum RBP4 level could inhibit lipofuscin without inhibiting the visual cycle.