Conservation of expression without conservation of regulatory sequences

Conservation is a reliable indicator of what sequence features have a function. Very often, in comparative genomics, conservation is the only clue available. However, there are many examples of highly conserved sequence for which no function can be identified, and many of these appear to be non-essential. A recent review by Weirauch and Hughes ("Conserved expression without conserved regulatory sequence: the more things change, the more they stay the same." Trends in Genetics 26:64, PMID 20083321) considers the opposite case: when expression is conserved but regulatory sequences are not. They list 17 examples of genes whose expression pattern is conserved despite divergent cis-regulatory sequences. In many of these cases the regulatory mechanisms are well known, and the review also includes a discussion of mechanisms that allow expression patterns to persist without conservation of the cis-regulatory signals.