Bovine studies (Beharry et al. 2004) have indicated NRPE to be the preferred substrate for ABCA4 (ABCR, rim protein, RmP) (Azarian et al. 1998), which acts as an inward-directed retinoid flippase and facilitates the transfer of NRPE to the cytoplasmic side of the disc membrane. This transfer is essential to avoid build up of potentially toxic retinoid intermediates which are implicated in many retinal degenerative diseases (see review Tsybovsky et al. 2010). Defective ABCA4 cannot perform this function leading to impaired vision and blindness disorders such as Stargardt disease (MIM:248200).