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: Conversion to rapamycin immunosuppression in renal transplant recipients : report of an initial experience .
### Output Text:
Conversion to <span class="chemical">rapamycin</span> immunosuppression in renal transplant recipients : report of an initial experience . 