Intergenic distances in human, chimp, cow, mouse, and rat genomes. Intergenic distances were recorded from RefSeq annotations in each genome. The data are plotted as a comparison of head-to-head, head-to-tail or tail-to-tail gene arrangements. Intergenic distances of 1,000 bp or less in the head-to-head gene arrangement represent bidirectional promoters. P-values shown in each plot denote the significance of the number of genes found in the head-to-head group compared to the other categories at a distance of 1,000 bp or less, using a Chi-square test of independence.