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Open access, freely available online Synopses of Research Articles Plus Ça Change: Gene Enhancers Upset Evolutionary Assumption DOI: 10.1371/journal.pbio.0030138 A standard evolutionary assumption functional roles in each species, the may be due to relatively small differences is that the DNA of closely related species structures of these enhancers differ, with in the two species’ enhancers that have should be more similar in both structure large deletions and insertions between accumulated since their evolutionary and function than that of more distantly transcription factor binding sites, as split. related ones. This parsimonious rule of well as other changes. Nonetheless, The results of this study indicate an thumb holds true across wide expanses within each species, the spatiotemporal important caveat about interpreting the of time and among widely divergent pattern of expression induced by S2E evolution of gene regulatory regions. As species, but it has exceptions. In this issue, is essentially identical, suggesting that, a complex functional unit that integrates Michael Ludwig and colleagues show despite their structural differences, they a host of signals, the S2E is likely to that one exception is in an enhancer might be functionally interchangeable. be under strong stabilizing selection, region of a key developmental gene in Since loss of S2E is lethal, the viability maintaining its output within narrow fruitfl ies of the genus Drosophila. Here, of the embryos that resulted from these limits. T