original papers NATURE | VOL 421 | 23 JANUARY 2003 | www.nature.com/nature © 2003 Nature Publishing Group 397 original papers 398 © 2003 Nature Publishing Group NATURE | VOL 421 | 23 JANUARY 2003 | www.nature.com/nature
original papers NATURE | VOL 421 | 23 JANUARY 2003 | www.nature.com/nature © 2003 Nature Publishing Group 397 original papers 398 © 2003 Nature Publishing Group NATURE | VOL 421 | 23 JANUARY 2003 | www.nature.com/nature