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The discovery of a single intron with aberrant splice boundaries in the primitive protozoan Giardia raised questions about the origins of splicing. In the September 19 Nature, Simpson et al. report the discovery of introns with canonical boundary sequences in the closely-related microbial eukaryote Carpediemonas membranifera (Nature 2002, 419:270). They analysed two distinct carbamate kinase genes from Carpediemonas and found short introns flanked by characteristic GT and AG sequences, at the 5' and 3' boundaries, respectively. The Giardia intron has a non-canonical CT dinucleotide at the 5' boundary. The authors conclude that "there is now every reason to assume that canonical introns were present in the most recent common ancestor of living eukaryotes."

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