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: Calcineurin inhibitors , such as cyclosporine and tacrolimus , have been available for almost 20 years .
### Output Text:
Calcineurin inhibitors , such as <span class="chemical">cyclosporine</span> and <span class="chemical">tacrolimus</span> , have been available for almost 20 years . 