In the disc membranes of photoreceptor outer segments, all-trans-retinal (atRAL) can spontaneously and reversibly react with phosphatidiylethanolamine (PE, cephalin; a phospholipid found in in membranes) to form a Schiff base product, N-retinylidene-phosphatidylethanolamine (NRPE) (Poincelot et al. 1969, Parish et al. 1998, Mata et al. 2000). Then, either NRPE is "flipped" over to the cytoplasmic side of disc membranes by ABCA4 where it can dissociate (thus releasing atRAL to re-enter the retinoid cycle) or it can condense with another atRAL to form a toxic diretinal compound. The route taken is determined by whether a functional ABCA4 protein is present or not.