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: Morphologic damage was variable but was proportional to the total cumulative DXR dose between 100 and 600 mg / m2 .
### Output Text:
Morphologic damage was variable but was proportional to the total cumulative <span class="chemical">DXR</span> dose between 100 and 600 mg / m2 . 