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: Mice with cardiomyocyte - restricted deletion of STAT3 exhibited worse cardiac function , higher levels of cardiomyocyte apoptosis , and a greater induction of Ku70 and Ku80 in response to DOX treatment during the acute stage when compared with control animals .
### Output Text:
Mice with cardiomyocyte - restricted deletion of STAT3 exhibited worse cardiac function , higher levels of cardiomyocyte apoptosis , and a greater induction of Ku70 and Ku80 in response to <span class="chemical">DOX</span> treatment during the acute stage when compared with control animals . 