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: Over expression of vascular endothelial growth factor and its receptor during the development of estrogen - induced rat pituitary tumors may mediate estrogen - initiated tumor angiogenesis .
### Output Text:
Over expression of vascular endothelial growth factor and its receptor during the development of <span class="chemical">estrogen</span> - induced rat pituitary tumors may mediate <span class="chemical">estrogen</span> - initiated tumor angiogenesis . 