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 become progressively refractory to conventional chemotherapy , and their prognosis is poor .
### Output Text:
Patients become progressively refractory to conventional chemotherapy , and their prognosis is poor . 