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: ILK / liver - / - mice also showed increased expression of key genes involved in hepatocyte proliferation at different time points during PB administration .
### Output Text:
ILK / liver - / - mice also showed increased expression of key genes involved in hepatocyte proliferation at different time points during <span class="chemical">PB</span> administration . 