Several developmental genes display remarkably conserved expression patterns between distantly related organisms. Little is known about the conservation of mechanisms that control gene expression, but it is perhaps surprising that enhancer sequences diverge relatively rapidly. On p. 5133, Ruvinsky and Ruvkun test whether enhancers from the fly can drive expression in homologous cell types in the worm. Approximately 90% of fly enhancers are unable to function in the worm, indicating that the binding sites of fly enhancer sequences and worm transcription factors have diverged too far for proper recognition to take place. The authors suggest that expression patterns are not conserved by preserving enhancer function, but by co-evolution among members of transcription initiation complexes, thus preserving the cohesive interactions between enhancers and their binding factors.