Correction for regional Alu density. Shown is the Alu content in flanking regions of human genes (20 kilobases) and expression profiles correcting for regional Alu density. Each point represents the Alu content in sliding windows of 1 kilobase (moving 200 base pairs at a time) after correcting for regional Alu density (Alu content in opposite flank of gene) through regression analysis (see Materials and methods). Groups represent the top 20% of genes with highest ('High'), 20% with the lowest ('Low), and 20% of medium ('Medium') breadth of expression. Points for high and low groups significantly different from medium expression levels (Student's t-tests using Bonferroni correction) are represented by closed circles.