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 clinical utility of FK 506 is complicated by substantial hypertension and nephrotoxicity .
### Output Text:
The clinical utility of <span class="chemical">FK 506</span> is complicated by substantial hypertension and nephrotoxicity . 