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Tortora G, Yokozaki H, Pepe S, Clair T, Cho-Chung YS.Tortora G, et al.Proc Natl Acad Sci U S A. 1991 Mar 1;88(5):2011-5. doi: 10.1073/pnas.88.5.2011.Proc Natl Acad Sci U S A. 1991.PMID: 2000408Free PMC article.