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Wang X, Xu B, Souza SC, Kopchick JJ.Wang X, et al. Among authors: kopchick jj.Proc Natl Acad Sci U S A. 1994 Feb 15;91(4):1391-5. doi: 10.1073/pnas.91.4.1391.Proc Natl Acad Sci U S A. 1994.PMID: 7509070Free PMC article.