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: METHODS AND RESULTS : To test whether clonidine elicits pharmacological effects independent of alpha2 - adrenoceptors , we have generated mice with a targeted deletion of all 3 alpha2 - adrenoceptor subtypes ( alpha2ABC - / - ) .
### Output Text:
METHODS AND RESULTS : To test whether <span class="chemical">clonidine</span> elicits pharmacological effects independent of alpha2 - adrenoceptors , we have generated mice with a targeted deletion of all 3 alpha2 - adrenoceptor subtypes ( alpha2ABC - / - ) . 