Mouse-human z-scores of motifs with experimentally verified ESS activity, which is deleterious for exons. Four of four of the Wang et al.  experimentally verified ESSs have negative z-scores, consistent with negative selection acting to remove them from coding regions. For the Zhang and Chasin  experimentally verified ESSs, nine of ten have negative z-scores (GTATTGTT, z = -0.006).
Kural et al. Genome Biology 2009 10:R133 doi:10.1186/gb-2009-10-11-r133