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: Development of clear cell adenocarcinoma in DES - exposed offspring under observation .
### Output Text:
Development of clear cell adenocarcinoma in <span class="chemical">DES</span> - exposed offspring under observation . 