Inclusion level of different groups. For each cassette exon, we counted the number of ESTs in which the exon is present (Np) or absent (Na), then calculated the inclusion level as Np/(Np+Na). The left column is for human and the right is for mouse. The frequencies of inclusion levels in ME, IND and LNK are quite different. Exons in LNK tend to be included often, which indicates that LNK transcripts with exon are the dominating form. Exons in IND are included at either a very high or very low level. Inclusion levels in ME are similar to IND, but there are more cases in which the transcripts with or without the cassette exon are express at similar levels. These results are observed in both mouse and human.