ArticleIssue Date1991CitationProceedings of the National Academy of Sciences of the United States of America, v.88, no.21, pp 9675 - 9679PublisherNational Academy of Sciences
ArticleIssue Date1992CitationProceedings of the National Academy of Sciences of the United States of America, v.89, no.19, pp 9166 - 9170PublisherNational Academy of Sciences