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 likely frequency of serious hepatotoxicity with sulfasalazine was estimated at 0 . 4 % of treated patients .
### Output Text:
The likely frequency of serious hepatotoxicity with <span class="chemical">sulfasalazine</span> was estimated at 0 . 4 % of treated patients . 