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: No ototoxic factor , other than DFO , was present in any of the patients .
### Output Text:
No ototoxic factor , other than <span class="chemical">DFO</span> , was present in any of the patients . 