We are grateful to E. Raviola and T. Reese for help with electron microscopy and to A. Jadhav and J. Trimarchi for access to unpublished data and reagents. Thanks to A. Jadhav, J. Trimarchi, D. Kim, and T. Cherry for helpful comments on the manuscript. This work was supported by the Howard Hughes Medical Institute and grants from the National Institutes of Health (EY014822 to JCC and EY009676 to CLC). Thanks to A. Swaroop for providing us with Nrl mutant mice. Competing interests. The authors have declared that no competing interests exist. Author contributions. JCC and CLC conceived and designed the experiments. JCC performed the experiments. JCC and CLC analyzed the data. JCC contributed reagents/materials/analysis tools. JCC and CLC wrote the paper. Abbreviations DAPI 6-diamidino-2-phenylindole E[number] embryonic day [number] ERG electroretinogram ESCS enhanced S-cone syndrome GCL ganglion cell layer INL inner nuclear layer ONL outer nuclear layer P[number] postnatal day [number]