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: The mean age of patients in the 16 probable cases was 57 . 9 , with hepatotoxicity being more common in women .
### Output Text:
The mean age of patients in the 16 probable cases was 57 . 9 , with hepatotoxicity being more common in women . 