To further examine whether Crx regulates mr-s transcription directly or not, we next performed a luciferase assay using the 1.2-kb proximal promoter region of mr-s fused to a luciferase gene as a luciferase reporter (Fig. 3D, Pro1.2k) and the Crx, Otx2, Nrl expression vectors, respectively. This 1.2-kb region of the mr-s upstream sequence contains three Crx binding consensus sequences. As shown in Fig. 3E, the luciferase activity was significantly upregulated when the Crx or Otx2 expression vector was co-introduced with Pro1.2k into HEK293T cells, while the luciferase activity was not altered when the Nrl expression vector was co-introduced. A previous report suggested that the transcriptional activity of Crx is augmented with Nrl when the rhodopsin promoter was used as a reporter gene [6]. On the other hand, our present data showed that the luciferase gene expression was not upregulated when both Crx and Nrl expression vectors were co-introduced with Pro1.2k compared to the activity when the Crx only expression vector was introduced. This may be due to cell type differences because a cell type of retinal/pineal origin was not used in our luciferase assay. In addition, our present data showed that Otx2, which is reported to have the same binding consensus as Crx, also transactivated mr-s expression. As shown in Fig. 2A–F, the expression pattern of mr-s correlates with that of Crx. In contrast, the transcripts of Otx2 are mainly detected in the photoreceptor layer at embryonic stages. Therefore, we concluded that mr-s transcription is directly regulated mainly by Crx.