U12-type introns are spliced by the U12-dependent spliceosome and are present in 
the genomes of many higher eukaryotic lineages including plants, chordates and 
some invertebrates. However, due to their relatively recent discovery and a 
systematic bias against recognition of non-canonical splice sites in general, 
the introns defined by U12-type splice sites are under-represented in genome 
annotations. Such under-representation compounds the already difficult problem 
of determining gene structures. It also impedes attempts to study these introns 
genome-wide or phylum-wide. The resource described here, the U12 Intron Database 
(U12DB), aims to catalog the U12-type introns of completely sequenced eukaryotic 
genomes in a framework that groups orthologous introns with each other. This 
will aid further investigations into the evolution and mechanism of 
U12-dependent splicing as well as assist ongoing genome annotation efforts. 
Public access to the U12DB is available at 
http://genome.imim.es/cgi-bin/u12db/u12db.cgi.