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: Since tamoxifen is widely used in breast cancer treatment and has been proposed for the prevention of breast cancer , its endometrial iatrogenic effects must be carefully examined .
### Output Text:
Since <span class="chemical">tamoxifen</span> is widely used in breast cancer treatment and has been proposed for the prevention of breast cancer , its endometrial iatrogenic effects must be carefully examined . 