In the Drosophila eye, sixteen different cell types are formed within a cluster of twenty-two cells that are present in each ommatidium. The fate of each cell is not determined by its lineage but by information that it receives from its neighbors. This information takes chemical form as a protein secreted from a cell that binds and activates a receptor protein on the surface of nearby cells, thereby triggering biochemical pathways inside the receiving cells and ultimately changing cell behavior. One such signaling protein is a ligand for the Drosophila EGF Receptor (EGFR). When EGFR binds its ligand, it triggers the RAS signal transduction pathway in a cell, which promotes its differentiation. Amazingly, this signal is needed to promote the differentiation of fifteen of the sixteen cell types in the ommatidium! How do various cells decode the presence of one signaling protein into 15 different instructions?