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: Verbal memory may be especially sensitive to changes in oestrogen levels , a finding commonly reported in studies of hormone replacement therapy in healthy women .
### Output Text:
Verbal memory may be especially sensitive to changes in <span class="chemical">oestrogen</span> levels , a finding commonly reported in studies of hormone replacement therapy in healthy women . 