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: MR imaging with quantitative diffusion mapping of tacrolimus - induced neurotoxicity in organ transplant patients .
### Output Text:
MR imaging with quantitative diffusion mapping of <span class="chemical">tacrolimus</span> - induced neurotoxicity in organ transplant patients . 