During Drosophila eye development, a single founder photoreceptor(R8) is specified in the eye imaginal disc for each of the compound eye's structural units. However, groups of disc cells express the proneural gene atonal (ato), which is required for eye development, so what restricts the R8 potential to single cells? On p. 4071, Pepple and co-workers propose a new two-step model to explain this mysterious process. The researchers show that ectopic R8s develop from R2 and R5 photoreceptor precursors independently of ectopic Ato in rough mutants(rough encodes a transcription factor that represses ato)and that Rough normally represses the R8-specific transcription factor senseless (sens) in these two precursors. Because R8 differentiation requires the repression of rough by Sens, these results suggest that sens activation by Ato and lateral inhibition together establish a transient pattern of R8s that is `locked' by a repressive feedback loop between...

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