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: Over a range of 1 - 150 days of DES treatment , pairs of control and DES - treated rats were sacrificed , and their pituitaries dissociated enzymatically into single - cell preparations .
### Output Text:
Over a range of 1 - 150 days of <span class="chemical">DES</span> treatment , pairs of control and <span class="chemical">DES</span> - treated rats were sacrificed , and their pituitaries dissociated enzymatically into single - cell preparations . 