Fig. 1.—

Cn and Nc nucleotides within SSs of Homo sapiens. For an intron to be spliced, it requires a pair of functional SSs that involve several nucleotides at the exon–intron boundaries. Both the donor and the acceptor SSs have intronic and exonic parts. The intronic parts (positions +1 … +6 of the donor SSs, and positions −24 … −1 of the acceptor SSs) are longer than the exonic parts (positions −3 … −1 of the donor SSs and position +1 of the acceptor SS). The height of the letters is proportional to the frequency of the corresponding nucleotide at this position. For each SS position, one or two frequent nucleotides were defined as Cn (blue), and the remaining nucleotides as Nc (black). The nucleotide frequencies in Drosophila melanogaster are very similar, and the same Cn nucleotides were defined. Sequence logos were constructed using WebLogo (Crooks et al. 2004).