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: The high tumor angiogenic potential was associated with an elevated VEGF / VPF protein expression in the E2 exposed pituitary of ovariectomized ( OVEX ) rats .
### Output Text:
The high tumor angiogenic potential was associated with an elevated VEGF / VPF protein expression in the <span class="chemical">E2</span> exposed pituitary of ovariectomized ( OVEX ) rats . 