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: The side effects of azathioprine include anemia , which has been attributed to bone marrow suppression .
### Output Text:
The side effects of <span class="chemical">azathioprine</span> include anemia , which has been attributed to bone marrow suppression . 