Spliced-leader (SL) trans-splicing is an essential step in pre-mRNA maturation in a variety of lower eukaryotic organisms. However, this processing pathway is absent in mammals, insects, yeast and plants. The patchy phylogenetic distribution of SL trans-splicing is consistent with either ‘multiple gain' or ‘multiple loss' evolutionary scenarios. Recent studies show that two additional metazoan phyla carry out SL trans-splicing, significantly increasing its phylogenetic range. However, it remains unclear whether this unusual type of splicing is an ancestral or an acquired trait.