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: Characterization of estrogen - induced adenohypophyseal tumors in the Fischer 344 rat .
### Output Text:
Characterization of <span class="chemical">estrogen</span> - induced adenohypophyseal tumors in the Fischer 344 rat . 