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: PATIENTS AND METHODS : 29 patients were enrolled on a study of thalidomide using an intra - patient dose escalation schedule .
### Output Text:
PATIENTS AND METHODS : 29 patients were enrolled on a study of <span class="chemical">thalidomide</span> using an intra - patient dose escalation schedule . 