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: Alpha2ABC - / - mice were completely unresponsive to the analgesic and hypnotic effects of clonidine ; however , clonidine significantly lowered heart rate in alpha2ABC - / - mice by up to 150 bpm .
### Output Text:
Alpha2ABC - / - mice were completely unresponsive to the analgesic and hypnotic effects of <span class="chemical">clonidine</span> ; however , <span class="chemical">clonidine</span> significantly lowered heart rate in alpha2ABC - / - mice by up to 150 bpm . 