Cell fate decisions are influenced by extrinsic and intrinsic factors, but understanding how these are integrated is key for determining how cell fate is specified during development. Here, Yannis Mavromatakis and Andrew Tomlinson use developing ommatidia of the Drosophila eye as a model for exploring the logic of cell fate decisions (p. 4353). The ommatidia are constructed in two distinct waves. It is known that the transcription factor Lozenge (Lz) is expressed in second wave cells and that Notch/receptor tyrosine kinase (N/RTK) signalling is involved in specifying each of the cell fates generated in the second wave. In this study, the researchers ectopically express Lz in first wave cells, supply them with appropriate N/RTK codes, and thereby reproduce each of the second wave cell fates. Based on the dissection of this series of experiments, they conclude that Lz provides key intrinsic information to second wave cells. They also infer that N/RTK activities, in concert with Lz, are only required for a short period of time to ‘lock in’ cell fate.