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: Estrogens , which have been associated with several types of human and animal cancers , can induce tumor angiogenesis in the pituitary of Fischer 344 rats .
### Output Text:
<span class="chemical">Estrogens</span> , which have been associated with several types of human and animal cancers , can induce tumor angiogenesis in the pituitary of Fischer 344 rats . 