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: At detection of reduction in sural nerve SAP amplitude , the median thalidomide cumulative dose was 21 . 4 g .
### Output Text:
At detection of reduction in sural nerve SAP amplitude , the median <span class="chemical">thalidomide</span> cumulative dose was 21 . 4 g . 