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: Thalidomide has limited single - agent activity in relapsed or refractory indolent non - Hodgkin lymphomas : a phase II trial of the Cancer and Leukemia Group B .
### Output Text:
<span class="chemical">Thalidomide</span> has limited single - agent activity in relapsed or refractory indolent non - Hodgkin lymphomas : a phase II trial of the Cancer and Leukemia Group B . 