Pre-mRNA splicing in eukaryotes is a fundamental step in gene expression and represents an important level at which the expression of protein-coding genes can be regulated . In higher eukaryotes, there are two classes of nuclear pre-mRNA introns. The most abundant class consists of U2-dependent introns (U2 introns), whereas the second rarer class (<0.4% of introns) consists of U12-dependent introns (U12 introns). U12 introns have been found in the nuclear genomes of vertebrates, plants, and insects. Introns belonging to these two distinct classes are spliced by two different spliceosomes: the major U2-type spliceosome and the less abundant U12-type spliceosome. Although the first U12 introns to be described had AT-AC terminal dinucleotides, the majority of U12-type introns contain GT-AG, and a small number contain other noncanonical terminal dinucleotides. We use statistical methods to discriminate the two classes of introns and investigate U12-introns evolutionaty distribution.