NEWS AND VIEWS A panorama of mammalian gene expression evolution

comparison of humans and chimps that the two species, despite having numerous phenotypic differences, are extre-mely similar in protein sequences. They speculated that changes in gene expression levels and patterns must have played a more important role in phenotypic evolution than changes in protein sequences and functions. However, testing King and Wilson’s hypothesis has not been easy, in part because of the difficulty in comparing gene expressions between species, especially at the genomic scale. Since B10 years ago, microarray has been used to quantify and compare genome-wide gene expressions across species, but this technology has serious limitations. First, if different probes are used to measure the expressions of orthologous genes in two species, the estimated expression levels are usually not directly comparable because of differential affinities o