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: INTERPRETATION : Although this preliminary analysis has low power , in this cohort of women at low - to - normal risk of breast cancer , the postulated protective effects of tamoxifen are not yet apparent .
### Output Text:
INTERPRETATION : Although this preliminary analysis has low power , in this cohort of women at low - to - normal risk of breast cancer , the postulated protective effects of <span class="chemical">tamoxifen</span> are not yet apparent . 