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: It is concluded that PB , which promotes carcinogenesis when administered after the DEN treatment , reduces the carcinogen effect when given simultaneously with DEN .
### Output Text:
It is concluded that <span class="chemical">PB</span> , which promotes carcinogenesis when administered after the <span class="chemical">DEN</span> treatment , reduces the carcinogen effect when given simultaneously with <span class="chemical">DEN</span> . 