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: BACKGROUND : Sirolimus is the latest immunosuppressive agent used to prevent rejection , and may have less nephrotoxicity than calcineurin inhibitor ( CNI ) - based regimens .
### Output Text:
BACKGROUND : <span class="chemical">Sirolimus</span> is the latest immunosuppressive agent used to prevent rejection , and may have less nephrotoxicity than calcineurin inhibitor ( CNI ) - based regimens . 