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: A case of bilateral optic neuropathy in a patient on tacrolimus ( FK506 ) therapy after liver transplantation .
### Output Text:
A case of bilateral optic neuropathy in a patient on <span class="chemical">tacrolimus</span> ( <span class="chemical">FK506</span> ) therapy after liver transplantation . 