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Clawson GA, Smuckler EA.Clawson GA, et al.Proc Natl Acad Sci U S A. 1978 Nov;75(11):5400-4. doi: 10.1073/pnas.75.11.5400.Proc Natl Acad Sci U S A. 1978.PMID: 281689Free PMC article.