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: The mechanistic details of tumor angiogenesis induction , during estrogen carcinogenesis , are still unknown .
### Output Text:
The mechanistic details of tumor angiogenesis induction , during <span class="chemical">estrogen</span> carcinogenesis , are still unknown . 