Correlation of Rat and Mouse Responses to TCDD. To assess the variability of the response to TCDD between mice and rats we took the pre-processed and linearly-modeled data and selected all ProbeSets with evidence for differential mRNA abundances (padjusted < 0.01) in at least one species. We mapped homologs between the two species using the Homologene database. A) To determine if genes showed similar trends in their profiles we plotted the fold-change in log2 space (M-values) for all homologs. The two profiles are well-correlated (Spearman's rho = 0.32, p < 2.2 × 10-16), showing similar trends. B) To determine if these similarities were the strongest pattern within the dataset we selected all genes with evidence for TCDD-induced changes in mRNA abundance (padjusted < 0.01 in one or both species) and selected the pre-processed values for all animals in both species. These values were then median-centered and root-mean-square-scaled separately within species before being subjected to divisive hierarchical clustering using the DIANA algorithm. The rat (pink) and mouse (red) TCDD animals cluster together, separately from the rat (light blue) and mouse (dark blue) controls. This unsupervised clustering suggests that species differences are less prominent than conserved TCDD-induced changes in mRNA abundances.