At the stellate cell surface, all-trans-retinol (atROL) binds to retinol binding protein 4 (RBP4, holo-RBP) (Kanai et al. 1968). atROL is insoluble in aqueous conditions and it's thought RBP4 picks up atROL from the outer leaflet of the plasma membrane. RBP4 is a 21 kDa protein secreted into the bloodstream by the liver in an atROL-dependent manner. Defects in RBP4 cause retinol-binding protein deficiency (RBP deficiency, MIM:180250), causing night vision problems and progressive atrophy of the retinal pigment epithelium (RPE) (Seeliger et al. 1999).