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: Genetic separation of tumor growth and hemorrhagic phenotypes in an estrogen - induced tumor .
### Output Text:
Genetic separation of tumor growth and hemorrhagic phenotypes in an <span class="chemical">estrogen</span> - induced tumor . 