In developing photoreceptors, an extraordinary growth process occurs whereby the outer segment is generated from the nascent connecting cilium (see [38] and references therein). Peripherin/RDS and ROM-1 proteins (localized in disc rims) and the opsin proteins (localized throughout the discs) have important roles in the structural integrity of mature outer segments (see [39,29]). ROM-1-/- mice produce disorganized outer segments with large disks [40]. Crx, by virtue of being a transcription factor, presumably controls genes that are responsible for the building and perhaps maintenance of the outer segment structure, including rhodopsin and peripherin. Using northern blots [34], microarrays [10], and serial analysis of gene expression (SAGE) [35], we have defined a large number of genes that are altered in their expression level in Crx-/- mice. We found that rhodopsin expression was severely diminished in Crx-/- animals, and peripherin mRNA was reduced by approximately 30%. Transgenic mice with variable levels of expression of wild type rhodopsin exhibit rod degeneration [41], indicating the importance of the level of rhodopsin expression. In addition, the timing of rhodopsin expression may be very important, as indicated by studies in Drosophila.