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: Thus , CAA directly reacts with cellular protein and non - protein thiols , mediating its toxicity on hRPTEC .
### Output Text:
Thus , <span class="chemical">CAA</span> directly reacts with cellular protein and non - protein <span class="chemical">thiols</span> , mediating its toxicity on hRPTEC . 