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: To test the hypothesis that estrogen increases PRL - R expression and sensitivity to prolactin , we next demonstrated that estradiol greatly augments prolactin - induced STAT5 activation .
### Output Text:
To test the hypothesis that <span class="chemical">estrogen</span> increases PRL - R expression and sensitivity to prolactin , we next demonstrated that <span class="chemical">estradiol</span> greatly augments prolactin - induced STAT5 activation . 