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: We have investigated the association between endometrial cancer and tamoxifen use or other treatments in women treated for breast cancer in a case - control study .
### Output Text:
We have investigated the association between endometrial cancer and <span class="chemical">tamoxifen</span> use or other treatments in women treated for breast cancer in a case - control study . 