science?_ob=ArticleURL&_udi=B6VJX-4834NV6-1&_coverDate=04%2F30%2F2003&_alid=515922818&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6106&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=dc2e357fd02e909e7ebeb54f3d867221
science?_ob=ArticleURL&_udi=B6VJX-4834NV6-1&_coverDate=04%2F30%2F2003&_alid=515922818&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6106&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=dc2e357fd02e909e7ebeb54f3d867221