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: In conclusion , CNS complications are frequent events during ALL therapy , and require rapid detection and prompt treatment to limit permanent damage .
### Output Text:
In conclusion , CNS complications are frequent events during ALL therapy , and require rapid detection and prompt treatment to limit permanent damage . 