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: 4 cases of endometrial carcinoma referred from elsewhere demonstrated the problems of inappropriate and unsupervised unopposed oestrogen therapy and the difficulty in distinguishing severe hyperplasia from malignancy .
### Output Text:
4 cases of endometrial carcinoma referred from elsewhere demonstrated the problems of inappropriate and unsupervised unopposed <span class="chemical">oestrogen</span> therapy and the difficulty in distinguishing severe hyperplasia from malignancy . 