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: cTnI ( ng / ml ) , CK - MB mass and CK remained unchanged in DOX rats compared with controls .
### Output Text:
cTnI ( ng / ml ) , CK - MB mass and CK remained unchanged in <span class="chemical">DOX</span> rats compared with controls . 