International Standard Serial Number, the international code that uniquely identifies a serial publication title.
<!ELEMENT issn (#PCDATA %issn-elements;)* >
(#PCDATA)*
Text, numbers, or special characters, zero or more
<element-citation>, <journal-meta>, <mixed-citation>, <product>, <related-article>, <related-object>
...
<front>
<journal-meta>
<journal-id journal-id-type="pmc">BMJ</journal-id>
<journal-id journal-id-type="publisher">BR MED J</journal-id>
<issn>0959-8138</issn>
<publisher>
<publisher-name>British Medical Journal</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="pmid">10092260</article-id>
<title-group>
<article-title>Systematic review of day hospital
care for elderly people</article-title>
</title-group>
<contrib-group>...</contrib-group>
<aff id="StLukes">...</aff>
<pub-date pub-type="pub" iso-8601-date="1999-03-27">
...</pub-date>
...
</article-meta>
</front>
...
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