Sequence conservation in bidirectional promoters. The 44-way vertebrate phyloP scores are calculated in intergenic regions collected from head-to-head, head-to-tail, or tail-to-tail gene arrangements. Coding regions and ancestral repeats serve as control sets for comparison. These data are plotted as the percentage of bases having phyloP scores equal to or larger than the phyloP score specified on the horizontal axis. Negative scores indicate accelerated change whereas positive scores indicate sequence conservation.