Introns containing weak PY tracts are enriched in specific motifs upstream of the PY tract. Shown are representative motifs derived from n-mers enriched in the region upstream of weak PY tracts (see Materials and methods for details of motif construction). The complete list of motifs is available in Additional data files 4 and 5. The average Z-score for enrichment of all of the n-mers that compose the motif is shown to the right. (a) Motifs over-represented upstream of weak PY tracts for GC-rich human introns. (b) Motifs over-represented upstream of weak PY tracts for AT-rich human introns.