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Beck G, Habicht GS.Beck G, et al. Among authors: habicht gs.Proc Natl Acad Sci U S A. 1986 Oct;83(19):7429-33. doi: 10.1073/pnas.83.19.7429.Proc Natl Acad Sci U S A. 1986.PMID: 3489938Free PMC article.