The retinal determination gene network (RDGN). Members of the RDGN are indicated on a yellow background. Toy activates expression of ey as well as so (reviewed in ). Ey activates expression of so, eya*, optix* and shf*  (boxed genes are direct Ey targets; asterisked genes were isolated by Ostrin et al. ). Eya and So interact to directly activate expression of their targets: lz, hh, ey and so (direct targets are underlined), as well as indirectly activating expression of the downstream gene dac [14,16,17]. Evidence suggests that Shf regulates the activity of Hh, and that Hh regulates eya expression [8,9,15]. Members of the RDGN are required for eye specification, upstream of photoreceptor specification and differentiation, with ato functioning to regulate specification of the first photoreceptor cell, R8 [3,4]. Protein-protein interactions are depicted by blue arrows. Red arrows indicate direct transcriptional regulation. Black arrows indicate an undetermined level of regulation. The position of the retinal determination gene eyg in the network is not yet clear, and so it is not included in this figure.