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 began thalidomide at 400 mg / d and escalated as tolerated to 1200 mg / d by day 54 .
### Output Text:
Patients began <span class="chemical">thalidomide</span> at 400 mg / d and escalated as tolerated to 1200 mg / d by day 54 . 