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: In DXR - CM myocardial damage is proportional to the degree of cytotoxic insult ( DXR dose ) while myocardial function is preserved until a critical dose or degree of damage is reached , after which myocardial performance deteriorates rapidly .
### Output Text:
In <span class="chemical">DXR</span> - CM myocardial damage is proportional to the degree of cytotoxic insult ( <span class="chemical">DXR</span> dose ) while myocardial function is preserved until a critical dose or degree of damage is reached , after which myocardial performance deteriorates rapidly . 