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: This case presented a unique opportunity to observe the histologic features of clinically reversible hepatic veno - occlusive disease over time , and may be the first case of veno - occlusive related solely to 6 - thioguanine .
### Output Text:
This case presented a unique opportunity to observe the histologic features of clinically reversible hepatic veno - occlusive disease over time , and may be the first case of veno - occlusive related solely to <span class="chemical">6 - thioguanine</span> . 