A number or expression that is set lower than the baseline and slightly smaller, to act as an inferior or subscript
<!ELEMENT sub (#PCDATA %emphasized-text;)* >
(#PCDATA | hr | email | ext-link | multi-link | uri | inline-supplementary-material | related-article | related-object | bold | italic | monospace | overline | overline-start | overline-end | roman | sans-serif | sc | strike | underline | underline-start | underline-end | alternatives | inline-graphic | private-char | chem-struct | inline-formula | tex-math | mml:math | abbrev | milestone-end | milestone-start | named-content | styled-content | fn | target | xref | sub | sup)*
Any combination of:
<addr-line>, <aff>, <alt-title>, <anonymous>, <article-title>, <attrib>, <award-id>, <bold>, <book-title>, <chapter-title>, <chem-struct>, <collab>, <collection-name>, <comment>, <compound-kwd-part>, <conf-loc>, <conf-name>, <conf-sponsor>, <copyright-holder>, <copyright-statement>, <corresp>, <def-head>, <degrees>, <disp-formula>, <edition>, <element-citation>, <etal>, <ext-link>, <fax>, <funding-source>, <funding-statement>, <given-names>, <gov>, <inline-formula>, <inline-supplementary-material>, <institution>, <issue>, <issue-part>, <issue-title>, <italic>, <kwd>, <label>, <license-p>, <meta-value>, <mixed-citation>, <monospace>, <named-content>, <on-behalf-of>, <overline>, <p>, <part-title>, <patent>, <phone>, <prefix>, <preformat>, <product>, <publisher-loc>, <publisher-name>, <related-article>, <related-object>, <role>, <roman>, <sans-serif>, <sc>, <series>, <sig>, <sig-block>, <source>, <std>, <strike>, <styled-content>, <sub>, <subject>, <subtitle>, <suffix>, <sup>, <supplement>, <surname>, <target>, <td>, <term>, <term-head>, <textual-form>, <th>, <title>, <trans-source>, <trans-subtitle>, <trans-title>, <underline>, <uri>, <verse-line>, <volume>, <volume-id>, <volume-series>, <xref>
An ordinary subscript:
... <p>... Apparently, the names of “FMR” genes do not reflect common sequence features but rather a physiological condition, “fragile X mental retardation” syndrome, associated with this gene. In this sense, the two are members of a group or family, but they show sequence similarity only in the pathological trinucleotide repeats (CGG)<sub>n</sub> that are often found upstream of their coding regions.</p> ...
As shown formatted above, a subscript and superscript in a “stacked ” arrangement:
... <p>The <italic>Escherichia coli argU</italic> gene encodes a minor arginine tRNA, tRNA<sup arrange="stack">Arg</sup><sub arrange="stack">UCU</sub>, which corresponds to the rare codon AGA and possibly AGG ...</p> ...