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: Young women on replacement estrogens for ovarian failure after cancer therapy may also have increased risk of endometrial carcinoma and should be examined periodically .
### Output Text:
Young women on replacement <span class="chemical">estrogens</span> for ovarian failure after cancer therapy may also have increased risk of endometrial carcinoma and should be examined periodically . 