Nrl is required for rod photoreceptor development.

The protein neural retina leucine zipper (Nrl) is a basic motif-leucine zipper transcription factor that is preferentially expressed in rod photoreceptors 1,2 . It acts synergistically with Crx to regulate rhodopsin transcription 3-5 . Missense mutations in human NRL have been associated with autosomal dominant retinitis pigmentosa 6,7 . Here we report that deletion of Nrl in mice results in the complete loss of rod function and super-normal cone function, mediated by S cones. The photoreceptors in the Nrl -/- retina have cone-like nuclear morphology 8 and short, sparse outer segments with abnormal disks. Analysis of retinal gene expression confirms the apparent functional transformation of rods into S cones in the Nrl -/- retina. On the basis of these findings, we postulate that Nrl acts as a 'molecular switch' during rod-cell development by directly modulating rod-specific genes while simultaneously inhibiting the S-cone pathway through the activation of Nr2e3.