Model of TH signaling in retinal development. In mitotic progenitor cells, Dio3 is expressed throughout the cell cycle at various periods of development. In mitotic cells that are about to produce photoreceptor cells, the indicated genes are expressed in G2. As the cell divides, it is likely that one daughter of the division becomes a photoreceptor cell, and retains expression of TRb for some time, while the other daughter re-enters cell cycle and quickly turns off TRb. In photoreceptor cells, as TRb expression decreases, Crx and visinin are expressed, and a small subset of cells, possibly rods, transiently expresses Dio2.