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: This prompted us to study the brain expression of P - glycoprotein ( Pgp ) , a major drug efflux transporter in many tissues , including the BBB , in TR ( - ) rats compared with nonmutant ( wild - type ) Wistar rats .
### Output Text:
This prompted us to study the brain expression of P - glycoprotein ( Pgp ) , a major drug efflux transporter in many tissues , including the BBB , in TR ( - ) rats compared with nonmutant ( wild - type ) Wistar rats . 