May determine the nasotemporal axis of the retina, and consequently specify the topographical projection of the retinal ganglion-cell axons to the tectum by controlling expression of their target genes. Retina and brain. Can be detected in regions including primordial retina and neuroepithelium by embryonic day 2 (E2). At E3, expressed in the nasal retina and pigment epithelium as well as in the telencephalon, and at E7 is expressed in retinal ganglion cells. Levels begin to decline from E4 and almost disappear by E10.