Identification of RAG-like transposons in protostomes suggests their ancient bilaterian origin.

Background: V(D) J recombination is essential for adaptive immunity in jawed vertebrates and is initiated by the RAG1-RAG2 endonuclease. The and genes are thought to have evolved from a (RAG-like) transposon containing convergently-oriented () and genes. Elements resembling this presumptive evolutionary precursor have thus far only been detected convincingly in deuterostomes, leading to the model that the transposon first appeared in an early deuterostome.