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: We have recently shown that estrogen negatively modulates the hypotensive effect of clonidine ( mixed alpha2 - / I1 - receptor agonist ) in female rats and implicates the cardiovascular autonomic control in this interaction .
### Output Text:
We have recently shown that <span class="chemical">estrogen</span> negatively modulates the hypotensive effect of <span class="chemical">clonidine</span> ( mixed alpha2 - / I1 - receptor agonist ) in female rats and implicates the cardiovascular autonomic control in this interaction . 