A single transcription factor can often be the target of multiple signaling cascades. For example, in Drosophila, the transcription factor Jun is phosphorylated by two different MAP kinases, JNK and ERK. Jun forms a heterodimer with another transcription factor called Fos, but it has not been clear whether Fos is also a target of both MAP kinases or whether it is merely a passive cofactor. Ciapponi et al. demonstrate in biochemical and genetic studies that Fos is indeed the target of multiple MAP kinases. Residues that are phosphorylated by ERK or JNK appear to specify Fos regulation of photoreceptor cell differentiation or ommatidial rotation, respectively, in the same cell of the developing Drosophila eye. The authors speculate that modification by a specific enzyme to regulate downstream biological events may be a general mechanism for molecules that are the target of multiple signaling pathways.