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: The molecular mechanisms of CsA and FK506 toxicity were investigated .
### Output Text:
The molecular mechanisms of <span class="chemical">CsA</span> and <span class="chemical">FK506</span> toxicity were investigated . 