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: Increased frequency of venous thromboembolism with the combination of docetaxel and thalidomide in patients with metastatic androgen - independent prostate cancer .
### Output Text:
Increased frequency of venous thromboembolism with the combination of <span class="chemical">docetaxel</span> and <span class="chemical">thalidomide</span> in patients with metastatic androgen - independent prostate cancer . 