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: Our results suggest that oxidative damage is associated with acute cardiotoxicity induced by a single dose of DOX only .
### Output Text:
Our results suggest that oxidative damage is associated with acute cardiotoxicity induced by a single dose of <span class="chemical">DOX</span> only . 