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: Furthermore , immunohistochemical studies demonstrated that VEGFR - 2 ( flk - 1 / KDR ) , expression was elevated significantly in the endothelial cells of microblood vessels after 7 days of E2 exposure .
### Output Text:
Furthermore , immunohistochemical studies demonstrated that VEGFR - 2 ( flk - 1 / KDR ) , expression was elevated significantly in the endothelial cells of microblood vessels after 7 days of <span class="chemical">E2</span> exposure . 