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 CBZ levels measured in plasma and brain of these animals , however , were below those normally considered protective .
### Output Text:
The <span class="chemical">CBZ</span> levels measured in plasma and brain of these animals , however , were below those normally considered protective . 