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: Wistar rats ( 180 g ) were treated by DEN alone or by DEN + PB during 2 , 4 and 6 weeks according to our schedule for hepatocarcinogenesis .
### Output Text:
Wistar rats ( 180 g ) were treated by <span class="chemical">DEN</span> alone or by <span class="chemical">DEN</span> + <span class="chemical">PB</span> during 2 , 4 and 6 weeks according to our schedule for hepatocarcinogenesis . 