In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Her aminotransferase levels returned to normal by postoperative day 23 , and her 2 - year follow - up showed no adverse events .
### Output Text:
Her aminotransferase levels returned to normal by postoperative day 23 , and her 2 - year follow - up showed no adverse events . 