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: Reversible cerebral lesions associated with tiazofurin usage : MR demonstration .
### Output Text:
Reversible cerebral lesions associated with <span class="chemical">tiazofurin</span> usage : MR demonstration . 