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: FK 506 decreased eNOS activity and the levels of eNOS mRNA in the aorta ( 48 % and 55 % , respectively ) .
### Output Text:
<span class="chemical">FK 506</span> decreased eNOS activity and the levels of eNOS mRNA in the aorta ( 48 % and 55 % , respectively ) . 