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: One week after the last DOX treatment ( acute stage ) , MHC - CB7 mice exhibited improved cardiac function and lower levels of cardiomyocyte apoptosis when compared with the NON - TXG mice .
### Output Text:
One week after the last <span class="chemical">DOX</span> treatment ( acute stage ) , MHC - CB7 mice exhibited improved cardiac function and lower levels of cardiomyocyte apoptosis when compared with the NON - TXG mice . 