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: Thirteen patients with acute leukemia were treated with a DNR - containing regimen .
### Output Text:
Thirteen patients with acute leukemia were treated with a <span class="chemical">DNR</span> - containing regimen . 