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: The donor - derived GFP ( + ) cells were rarely found in the brains of transplanted epileptic rats .
### Output Text:
The donor - derived GFP ( + ) cells were rarely found in the brains of transplanted epileptic rats . 