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: Surprisingly , by 13 weeks following the last DOX treatment ( late stage ) , MHC - CB7 exhibited a progressive decrease in cardiac function and higher rates of cardiomyocyte apoptosis when compared with NON - TXG mice .
### Output Text:
Surprisingly , by 13 weeks following the last <span class="chemical">DOX</span> treatment ( late stage ) , MHC - CB7 exhibited a progressive decrease in cardiac function and higher rates of cardiomyocyte apoptosis when compared with NON - TXG mice . 