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: All these effects of DES were more pronounced among previously ovariectomized animals .
### Output Text:
All these effects of <span class="chemical">DES</span> were more pronounced among previously ovariectomized animals . 