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: Our results suggest a causal role of tamoxifen in endometrial cancer , particularly when used as currently proposed for breast cancer prevention .
### Output Text:
Our results suggest a causal role of <span class="chemical">tamoxifen</span> in endometrial cancer , particularly when used as currently proposed for breast cancer prevention . 