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: After thalidomide treatment , sural SAP amplitude recovered in 3 patients .
### Output Text:
After <span class="chemical">thalidomide</span> treatment , sural SAP amplitude recovered in 3 patients . 