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 one patient , reintroduction of FK506 led to rapid recurrence of MAHA .
### Output Text:
In one patient , reintroduction of <span class="chemical">FK506</span> led to rapid recurrence of MAHA . 