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 dose - response relationship , if any , of thalidomide for renal cell carcinoma is unclear .
### Output Text:
The dose - response relationship , if any , of <span class="chemical">thalidomide</span> for renal cell carcinoma is unclear . 