U12-dependent introns are found in small numbers in most eukaryotic genomes, but 
their scarcity makes accurate characterisation of their properties challenging. 
A computational search for U12-dependent introns was performed using the draft 
version of the human genome sequence. Human expressed sequences confirmed 404 
U12-dependent introns within the human genome, a 6-fold increase over the total 
number of non-redundant U12-dependent introns previously identified in all 
genomes. Although most of these introns had AT-AC or GT-AG terminal 
dinucleotides, small numbers of introns with a surprising diversity of termini 
were found, suggesting that many of the non-canonical introns found in the human 
genome may be variants of U12-dependent introns and, thus, spliced by the minor 
spliceosome. Comparisons with U2-dependent introns revealed that the 
U12-dependent intron set lacks the 'short intron' peak characteristic of 
U2-dependent introns. Analysis of this U12-dependent intron set confirmed 
reports of a biased distribution of U12-dependent introns in the genome and 
allowed the identification of several alternative splicing events as well as a 
surprising number of apparent splicing errors. This new larger reference set of 
U12-dependent introns will serve as a resource for future studies of both the 
properties and evolution of the U12 spliceosome.