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Pena HB, de Souza CP, Simpson AJ, Pena SD.Pena HB, et al. Among authors: simpson aj.Proc Natl Acad Sci U S A. 1995 Jan 31;92(3):915-9. doi: 10.1073/pnas.92.3.915.Proc Natl Acad Sci U S A. 1995.PMID: 7846078Free PMC article.