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 this work the effect of CAA on human proximal tubule cells in primary culture ( hRPTEC ) was investigated .
### Output Text:
In this work the effect of <span class="chemical">CAA</span> on human proximal tubule cells in primary culture ( hRPTEC ) was investigated . 