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 mechanisms of this ticlopidine - induced cholestasis are unclear .
### Output Text:
The mechanisms of this <span class="chemical">ticlopidine</span> - induced cholestasis are unclear . 