represses short-wavelength cone genes to preserve long-wavelength cone and rod photoreceptor identity.

The role of transcription factors in photoreceptor gene regulation is fairly well understood, but knowledge of the cell-type-specific function of transcriptional cofactors remains incomplete. Here, we show that the transcriptional corepressor promotes rod differentiation and represses short-wavelength cone genes in long-wavelength cones in zebrafish. In retinas, red cones are transformed into hybrid red/ultraviolet (UV) cones, green cones are absent, the number of blue cones is approximately doubled, and the number of rods is greatly reduced. We also find that mouse represses S-opsin expression in dorsal M-cones-analogous to its role in repressing UV cone genes in zebrafish red cones. Thus, plays a key role in ensuring appropriate patterns of gene expression in rods and cone subtypes of both zebrafish and mice.