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: An F1 hybrid of F344 and BN exhibited significant pituitary growth after 10 weeks of DES treatment with an average mass of 26 . 3 + / - 0 . 7 mg compared with 8 . 6 + / - 0 . 9 mg for untreated rats .
### Output Text:
An F1 hybrid of F344 and BN exhibited significant pituitary growth after 10 weeks of <span class="chemical">DES</span> treatment with an average mass of 26 . 3 + / - 0 . 7 mg compared with 8 . 6 + / - 0 . 9 mg for untreated rats . 