All autonomous non-long terminal repeat (non-LTR) retrotransposons reported to 
date in vertebrates encode an apurinic/apyrimidinic endonuclease-like enzyme 
necessary for target sequence cleavage and subsequent target-primed reverse 
transcription. We describe here vertebrate non-LTR retrotransposons encoding 
another type of endonuclease more related to type IIS restriction enzymes. Such 
retrotransposons have been detected until now only in trypanosomes, nematodes, 
and arthropods. The retrotransposon Rex6 was identified in the genome of several 
teleost fish including Xiphophorus maculatus (platyfish), Oryzias latipes 
(medakafish), Oreochromis niloticus (Nile tilapia), and Fugu rubripes (Japanese 
pufferfish). Rex6 encodes a reverse transcriptase and a putative restriction 
enzyme-like endonuclease and is a member of the R4 family of non-LTR 
retrotransposons containing the Dong and R4 elements found in nematodes and 
insects. Rex6 was active in many species during teleost evolution and underwent 
several bursts of retrotransposition (some of them being relatively recent) 
leading to a high copy number of Rex6 in the genome of numerous fish. Extremely 
truncated Rex6-related sequences were detected by database screening in 
reptiles, including the snake Trimeresus flavoviridis and the lizard Anolis 
carolinensis, but not in sequences from the human genome project, suggesting 
that this element might have been lost from certain vertebrate lineages.