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: CASE : A 52 - year - old postmenopausal woman was treated with tamoxifen for stage II estrogen receptor - positive breast carcinoma .
### Output Text:
CASE : A 52 - year - old postmenopausal woman was treated with <span class="chemical">tamoxifen</span> for stage II <span class="chemical">estrogen</span> receptor - positive breast carcinoma . 