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: Phenobarbital ( PB ) has a reputation for safety , and it is commonly believed that PB - related increases in serum aminotransferase levels do not indicate or predict the development of significant chronic liver disease .
### Output Text:
<span class="chemical">Phenobarbital</span> ( <span class="chemical">PB</span> ) has a reputation for safety , and it is commonly believed that <span class="chemical">PB</span> - related increases in serum aminotransferase levels do not indicate or predict the development of significant chronic liver disease . 