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: The present study investigated whether this effect of estrogen involves interaction with alpha2 - and / or I1 - receptors .
### Output Text:
The present study investigated whether this effect of <span class="chemical">estrogen</span> involves interaction with alpha2 - and / or I1 - receptors . 