Histogram of genes with increasing YY1 density. Each olfactory receptor gene was scanned for the number of YY1 binding sites found within its transcript. The combined score was divided by the gene length, resulting in the YY1 density for the given olfactory receptor gene. Note that the quality of the YY1 scores varies based on the match generated by the Position Weight Matrix, and thus the calculation of YY1 binding sites per 1 kb is approximate. Genes were ordered based on increasing YY1 density and the histograms were derived by the relative position of a gene within the group of all genes. A) The mouse non-olfactory genes binned by relative order of increasing YY1 density (x-axis). The y-axis indicates the number of genes in each bin. B) Mouse olfactory genes binned in the same fashion, also from their position within the group of all genes. The highest group of bins corresponds to a density of 4-11 YY1 binding sites per 1 kb transcript for mouse though only 6 genes show a YY1 density corresponding to more than 8 YY1 sites per kb. For human and mouse (panels B&C), there is a large group of olfactory receptor genes with high YY1 density, whereas fish (D) does not show this pattern.