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: Our objective was to investigate brain MR imaging findings and the utility of diffusion - weighted ( DW ) imaging in organ transplant patients who developed neurologic symptoms during tacrolimus therapy .
### Output Text:
Our objective was to investigate brain MR imaging findings and the utility of diffusion - weighted ( DW ) imaging in organ transplant patients who developed neurologic symptoms during <span class="chemical">tacrolimus</span> therapy . 