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Phylogeny of mammals and previously identified syncytin genes. Mammals comprise the monotremes, the marsupials, and the eutherians, the latter comprising the four major clades: Afrotheria (I), Xenarthra (II), Euarchontoglires (III), and Laurasiatheria (IV) (data from ref. 1). Branch length is proportional to time (in My), and the time of insertion of the different syncytins identified to date is indicated with arrowheads.
