We found that Nile tilapia has undergone Liranaftate duplication of the vasa gene by an unusual mechanism, in which a large fragment encompassing the coding region was duplicated from the Silydianin original site and integrated in novel sites. Retention of the ancestral exonintron structure in the duplicated loci indicates the duplication was via a DNA intermediate, not by reverse transcription of an mRNA. The structure of the insertion in 72C07 suggests a circular intermediate in the duplication. Circular DNA intermediates have been recognized recently as a new mechanism to explain eukaryotic gene duplication. Borneman et al. characterized the genome of industrial strains of yeast Saccharomyces cerevisiae, and found a cluster of five ORFs have integrated into the genomes at multiple points via circular DNA intermediates, whose length is estimated to be around 15 kb. Durkin et al. also found a segment of the KIT gene that is involved in coloring animal coats, was duplicated via circular DNA intermediates, whose length is estimated to be less than 480 kb, and concluded that it would cause coat color changes in some breeds of cattle. Eichler et al. found the motif CAGGG near the breakpoints in duplicated human loci, and speculated that the motif would be evidence for duplication model by circular DNA intermediate. We could not find any similar motifs in the sequences of the duplication boundaries. However, an 8 bp inverted repeat was found in sites A and D of 38M07. We speculate that this 8 bp sequence was involved in generating circular DNA intermediates during the duplication. Apicomplexan parasites are responsible for some deadly parasitic diseases affecting humans and live stock. They comprise a wide range of unicellular eukaryotes among which Plasmodium falciparum and Toxoplasma gondii are the most serious threat to human health. T. gondii is responsible for encephalitis in immunocompromised individuals and birth defects in the offspring of infected mothers. The genetic tractability of T. gondii makes it a useful model for the study of apicomplexan parasites. The life cycle of T. gondii is complex with multiple differentiation steps that are critical to the survival of the parasite in human and feline hosts.