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. Can be detected in regions including primordial retina and neuroepithelium by embryonic day 2 (E2). At E3, expressed in the temporal retina and associated pigment epithelium as well as in part of the diencephalon, and at E7 is expressed in retinal ganglion cells. Levels begin to decline from E4 and almost disappear by E10.