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: In patients receiving tacrolimus therapy , blood concentration should be carefully controlled and extreme attention paid to cardiac involvement .
### Output Text:
In patients receiving <span class="chemical">tacrolimus</span> therapy , blood concentration should be carefully controlled and extreme attention paid to cardiac involvement . 