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: An open - label phase II study of low - dose thalidomide in androgen - independent prostate cancer .
### Output Text:
An open - label phase II study of low - dose <span class="chemical">thalidomide</span> in <span class="chemical">androgen</span> - independent prostate cancer . 