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