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 a 58 - year - old man receiving tacrolimus after orthotropic liver transplantation , serial neuro - ophthalmologic examinations and laboratory studies were performed .
### Output Text:
In a 58 - year - old man receiving <span class="chemical">tacrolimus</span> after orthotropic liver transplantation , serial neuro - ophthalmologic examinations and laboratory studies were performed . 