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: RESULTS : Eighteen of the DOX rats died prematurely of general toxicity during the 9 - week period .
### Output Text:
RESULTS : Eighteen of the <span class="chemical">DOX</span> rats died prematurely of general toxicity during the 9 - week period . 