original papers
398 NATURE | VOL 421 | 23 JANUARY 2003 | www.nature.com/nature© 2003 Nature Publishing Group
original papers
NATURE | VOL 421 | 23 JANUARY 2003 | www.nature.com/nature 399© 2003 Nature Publishing Group
original papers
400 NATURE | VOL 421 | 23 JANUARY 2003 | www.nature.com/nature© 2003 Nature Publishing Group
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