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: Mitochondrial DNA and its respiratory chain products are defective in doxorubicin nephrosis .
### Output Text:
Mitochondrial DNA and its respiratory chain products are defective in <span class="chemical">doxorubicin</span> nephrosis . 