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The adaptive immune system seems to have burst onto the evolutionary scene in a mere 50 million years, starting ~500 million years ago. Finding intermediate stages has proven difficult. In the June 20 Proceedings of the National Academy of Sciences Shintani et al. find that the lamprey, a jawless vertebrate reported to lack an adaptive immune system, expresses a member of the Spi family of lymphoid transcription factors (Proc. Natl. Acad. Sci. USA 2000, 97:7417-7422). Expression is limited to the ovary (inexplicably) and cells in the gut epithelium that morphologically resemble lymphocytes.