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: RESULTS : Immunofluorescence staining with the MRP2 antibody was found to label a high number of microvessels throughout the brain in normal Wistar rats , whereas such labeling was absent in TR ( - ) rats .
### Output Text:
RESULTS : Immunofluorescence staining with the MRP2 antibody was found to label a high number of microvessels throughout the brain in normal Wistar rats , whereas such labeling was absent in TR ( - ) rats . 