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: There were slightly increased proliferating cell nuclear antigen - positive cells in the ILK / liver - / - animals at day 2 as compared to WT after PB administration .
### Output Text:
There were slightly increased proliferating cell nuclear antigen - positive cells in the ILK / liver - / - animals at day 2 as compared to WT after <span class="chemical">PB</span> administration . 