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: Therefore we did a trial in hysterectomised women of tamoxifen as a chemopreventive .
### Output Text:
Therefore we did a trial in hysterectomised women of <span class="chemical">tamoxifen</span> as a chemopreventive . 