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: - Tacrolimus ( FK 506 ) is a powerful , widely used immunosuppressant .
### Output Text:
- <span class="chemical">Tacrolimus</span> ( <span class="chemical">FK 506</span> ) is a powerful , widely used immunosuppressant . 