Although interphotoreceptor retinoid-binding protein (RBP3, IRBP) (Fong & Bridges 1988, Fong et al. 1990) is not required to move all-trans-retinol (atROL) from photoreceptor cells to the retinal pigment epithelium (RPE), it may function to regulate retinoid trafficking and possibly protect retinoids from biochemical damage. RBP3 is secreted by photoreceptor cells into the interphotoreceptor matrix (IPM), where, being a larger protein (135kDa) than the IPM space, becomes trapped (see mini-review Gonzalez-Fernandez & Ghosh 2008). It is through this space that retinoids move between the RPE and photoreceptor outer segments during the retinoid cycle. Once atROL enters the RPE, it binds with RBP1.