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: After the end of the treatment , the number and the size of induced PAS positive preneoplastic foci was significantly reduced when PB was given simultaneously with DEN for 4 and 6 weeks .
### Output Text:
After the end of the treatment , the number and the size of induced PAS positive preneoplastic foci was significantly reduced when <span class="chemical">PB</span> was given simultaneously with <span class="chemical">DEN</span> for 4 and 6 weeks . 