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: PURPOSE : To report a case of bilateral optic neuropathy in a patient receiving tacrolimus ( FK 506 , Prograf ; Fujisawa USA , Inc , Deerfield , Illinois ) for immunosuppression after orthotropic liver transplantation .
### Output Text:
PURPOSE : To report a case of bilateral optic neuropathy in a patient receiving <span class="chemical">tacrolimus</span> ( <span class="chemical">FK 506</span> , Prograf ; Fujisawa USA , Inc , Deerfield , Illinois ) for immunosuppression after orthotropic liver transplantation . 