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: These findings suggest that over expression of VEGF and its receptor ( VEGFR - 2 ) may play an important role in the initial step of the regulation of estrogen induced tumor angiogenesis in the rat pituitary .
### Output Text:
These findings suggest that over expression of VEGF and its receptor ( VEGFR - 2 ) may play an important role in the initial step of the regulation of <span class="chemical">estrogen</span> induced tumor angiogenesis in the rat pituitary . 