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: Thromboxane levels were not significantly affected by any of the treatments , indicating that the effects seen were attributable to inhibition of COX - 2 , but not COX - 1 .
### Output Text:
<span class="chemical">Thromboxane</span> levels were not significantly affected by any of the treatments , indicating that the effects seen were attributable to inhibition of COX - 2 , but not COX - 1 . 