Fig. 5. Model of neuronal subtype specification in the retina by combinations of HD and bHLH proteins. Different combinations of bHLH proteins (purple) and HD proteins (red) are expressed by the different classes of neurons and glia in the retina. From mouse mutant analyses and from coexpression of HD proteins and bHLH proteins in mouse retinal explants, Hatakeyama and Kageyama have proposed that the combinatorial activity of different HD and bHLH proteins determines the fate of retinal cells. See text and Hatakeyama and Kageyama (Hatakeyama and Kageyama, 2004) for details. NeuroD (Neurod1).