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: Normalization was obtained by halving the estrogen dose .
### Output Text:
Normalization was obtained by halving the <span class="chemical">estrogen</span> dose . 