Transcription factors Otx2 and Crx are known to be key regulators of retinal photoreceptor development [3,8,9]. Although both Otx2 and Crx are expressed in developing photoreceptor cells in the embryonic retina, only Crx is highly expressed in the postnatal photoreceptors, suggesting that mr-s may be regulated by Crx. To test whether Crx regulates mr-s transcription, we performed in situ hybridization of mr-s mRNA on wild-type and Crx KO P5 retinas (Fig. 3A, B). In the Crx KO retina, the mr-s transcript was dramatically reduced (Fig. 3B). This indicates that Crx is essential for transactivation of mr-s. Moreover, to test whether the transcription of mr-s is also regulated by Crx in the pineal gland, RT-PCR analysis was used to compare transcriptional levels of mr-s in P28 wild-type and Crx KO pineal gland (Fig. 3C). The results revealed that mr-s transcription was significantly downregulated in the Crx KO pineal gland. Taken together, these data indicate that mr-s transcription is regulated by Crx in the developing photoreceptors and pineal gland.