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: TAC has been shown to be a potent immunosuppressive agent for solid organ transplantation in pediatrics .
### Output Text:
<span class="chemical">TAC</span> has been shown to be a potent immunosuppressive agent for solid organ transplantation in pediatrics . 