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: However , literature regarding the incidence of the recurrence of TMA in patients exposed sequentially to cyclosporine and tacrolimus is limited .
### Output Text:
However , literature regarding the incidence of the recurrence of TMA in patients exposed sequentially to <span class="chemical">cyclosporine</span> and <span class="chemical">tacrolimus</span> is limited . 