Densities and orientations of TEVs with respect to the transcriptional (sense) direction of mouse genes. (a) Orientation bias within first, middle and last introns of protein coding genes. All TEV types occur preferentially in the antisense orientation, with the ERV TEV bias being the strongest. ERV TEVs show a lower bias in the first introns of genes (P < 10-3 by chi-square test). SINE TEVs show a significantly stronger orientation bias in the first introns of protein coding genes (P < 10-3). (b) Orientation biases are not significantly different between 'young' and 'old' TEVs, categorized using percentage sequence divergence from the repeat consensus sequence (x-axis). *** indicates P < 10-3.