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: The mitotic inhibition and the production of micronuclei normally observed after partial hepatectomy in DEN treated rats were also significantly decreased in DEN + PB treated rats .
### Output Text:
The mitotic inhibition and the production of micronuclei normally observed after partial hepatectomy in <span class="chemical">DEN</span> treated rats were also significantly decreased in <span class="chemical">DEN</span> + <span class="chemical">PB</span> treated rats . 