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Schafer MH, Ferraro KF, Williams SR.Schafer MH, et al.J Gerontol A Biol Sci Med Sci. 2011 Jun;66(6):667-73. doi: 10.1093/gerona/glr009. Epub 2011 Feb 24.J Gerontol A Biol Sci Med Sci. 2011.PMID: 21350245