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 data extend the findings of other investigators , further establishing the DES - induced tumor as a model for study of PRL cellular control mechanisms .
### Output Text:
The data extend the findings of other investigators , further establishing the <span class="chemical">DES</span> - induced tumor as a model for study of PRL cellular control mechanisms . 