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: After 15 days of E2 exposure , VEGF / VPF protein expression , in the non - endothelial cell population , sharply declined and was restricted to the blood vessels .
### Output Text:
After 15 days of <span class="chemical">E2</span> exposure , VEGF / VPF protein expression , in the non - endothelial cell population , sharply declined and was restricted to the blood vessels . 