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Shukla GC, Padgett RA.Shukla GC, et al.Proc Natl Acad Sci U S A. 2004 Jan 6;101(1):93-8. doi: 10.1073/pnas.0304919101. Epub 2003 Dec 22.Proc Natl Acad Sci U S A. 2004.PMID: 14691257Free PMC article.