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: Laboratory results were compared between prior , during and following sirolimus use .
### Output Text:
Laboratory results were compared between prior , during and following <span class="chemical">sirolimus</span> use . 