Barrière et al used the conserved expression pattern of the GABA transporter unc-47 in the nematodes Caenorhabditis elegans and Caenorhabditis briggsae to investigate how gene expression patterns are conserved in genetically diverged species. By expressing the distinct cis-regulatory elements from one species in the other, the resultant altered expression patterns revealed the influence of the trans environment. The study showed that on rapid evolution of C. briggsae, compensatory mutations in cis elements and their associated trans factors maintained expression patterns.