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: At this point it was assumed that the cytochrome P450 / CCl4 toxic state was both maximal and stable .
### Output Text:
At this point it was assumed that the cytochrome P450 / <span class="chemical">CCl4</span> toxic state was both maximal and stable . 