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Metcalf D, Nicola NA.Metcalf D, et al. Among authors: nicola na.Proc Natl Acad Sci U S A. 1991 Jul 15;88(14):6239-43. doi: 10.1073/pnas.88.14.6239.Proc Natl Acad Sci U S A. 1991.PMID: 1712485Free PMC article.