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Paytubi S, McMahon SA, Graham S, Liu H, Botting CH, Makarova KS, Koonin EV, Naismith JH, White MF.Paytubi S, et al.Proc Natl Acad Sci U S A. 2012 Feb 14;109(7):E398-405. doi: 10.1073/pnas.1113277108. Epub 2011 Nov 21.Proc Natl Acad Sci U S A. 2012.PMID: 22106294Free PMC article.