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: Doxorubicin is an anti - tumor agent that represses cardiac - specific gene expression and induces myocardial cell apoptosis .
### Output Text:
<span class="chemical">Doxorubicin</span> is an anti - tumor agent that represses cardiac - specific gene expression and induces myocardial cell apoptosis . 