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: Beyond 8 days of DES exposure , the immunochemically PRL - positive proportion of cells increased to over 50 % of the total population .
### Output Text:
Beyond 8 days of <span class="chemical">DES</span> exposure , the immunochemically PRL - positive proportion of cells increased to over 50 % of the total population . 