The <isbn> element holds an International Standard Book Number. When an ISBN appears in a citation or product information, it should be tagged in an <isbn> element. The <isbn> element should contain only the ISBN. Any other text or label should be placed outside of the <isbn> element.
Correct:
<isbn>0-201-89683-4</isbn>
Incorrect:
<isbn>ISBN 0-201-89683-4</isbn>
Correct:
<ref id="CIT0012"><mixed-citation publication-type="book"><person-group person-group-type="author"><string-name><surname>Knuth</surname>, <given-names>D. E.</given-names></string-name></person-group> (<year>1997</year>). <source><italic>The art of computer programming, volume 1 (3rd Ed.): Fundamental algorithms</italic></source>. <publisher-loc>Redwood City</publisher-loc>: <publisher-name>Addison Wesley Longman</publisher-name>. ISBN <isbn>0-201-89683-4</isbn>.</mixed-citation></ref>
Incorrect:
<ref id="CIT0012"><mixed-citation publication-type="book"><person-group person-group-type="author"><string-name><surname>Knuth</surname>, <given-names>D. E.</given-names></string-name></person-group> (<year>1997</year>). <source><italic>The art of computer programming, volume 1 (3rd Ed.): Fundamental algorithms</italic></source>. <publisher-loc>Redwood City</publisher-loc>: <publisher-name>Addison Wesley Longman</publisher-name>. <isbn>ISBN 0-201-89683-4</isbn>.</mixed-citation></ref>