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: These data suggest that CS intervenes in a critical cellular event that is an important common pathway to toxic cell death .
### Output Text:
These data suggest that <span class="chemical">CS</span> intervenes in a critical cellular event that is an important common pathway to toxic cell death . 