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: MRP2 is absent in the transport - deficient ( TR ( - ) ) Wistar rat mutant , so that this rat strain was very helpful in defining substrates of MRP2 by comparing tissue concentrations or functional activities of compounds in MRP2 - deficient rats with those in transport - competent Wistar rats .
### Output Text:
MRP2 is absent in the transport - deficient ( TR ( - ) ) Wistar rat mutant , so that this rat strain was very helpful in defining substrates of MRP2 by comparing tissue concentrations or functional activities of compounds in MRP2 - deficient rats with those in transport - competent Wistar rats . 