Exons often contain specific short oligonucleotide sequences that affect their ability to be spliced. Exonic splicing enhancers (ESEs) within exons promote splicing of the corresponding exons and subsequent exon inclusion mediated by splicing regulatory proteins. The best-studied family of splicing regulatory proteins are Serine/ Arginine-rich proteins (SR proteins), which include the proteins SF2/ASF, SC35, SRp20, SRp30c and many others (31, 32) . It has become clear that many exons *To whom correspondence should be addressed. Tel: 4163404800 (ext. 6759); Fax: 4169289466; Email: zhiqi.chen@utoronto.ca contain ESE elements that bind to specific members of the SR family (25) , leading to exon inclusion.