Genome-Wide Mapping of Differential Expression. To determine if differentially-expressed ProbeSets were localized to specific portions of the rat or mouse genome we plotted the entire genome, with one chromosome per line. Each gene was plotted with a white bar representing its location on the chromosome and its position on the plus (up) or minus (down) strand. Genes showing differential abundances (padjusted < 0.01) in the rat are colour-coded in red, those in the mouse in blue, and those in both species in black. No prominent clusters of expression are observed in either the rat (A) or mouse (B).