Tuning in to the signals: noncoding sequence conservation in vertebrate genomes

Aligning and comparing genomic sequences enables the identification of conserved sequence signatures and can enrich for coding and noncoding functional regions. In vertebrates, the comparison of human and rodent genomes and the comparison of evolutionarily distant genomes, such as human and pufferfish, have identified specific sets of 'ultraconserved' sequence elements associated with the control of early development. However, is this just the tip of a 'conservation iceberg' or do these sequences represent a specific class of regulatory element? Studies on the zebrafish phox2b gene region and the ENCODE project suggest that many regulatory elements are not highly conserved, posing intriguing questions about the relationship between noncoding sequence conservation and function and the evolution of regulatory sequences.