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Simmons LA, Grossman AD, Walker GC.Simmons LA, et al. Among authors: walker gc.Proc Natl Acad Sci U S A. 2007 Jan 23;104(4):1360-5. doi: 10.1073/pnas.0607123104. Epub 2007 Jan 17.Proc Natl Acad Sci U S A. 2007.PMID: 17229847Free PMC article.