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: CONCLUSIONS : Direct inhibition of cardiac HCN pacemaker channels contributes to the bradycardic effects of clonidine gene - targeted mice in vivo , and thus , clonidine - like drugs represent novel structures for future HCN channel inhibitors .
### Output Text:
CONCLUSIONS : Direct inhibition of cardiac HCN pacemaker channels contributes to the bradycardic effects of <span class="chemical">clonidine</span> gene - targeted mice in vivo , and thus , <span class="chemical">clonidine</span> - like drugs represent novel structures for future HCN channel inhibitors . 