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: However , there is no research on GFC effects in the central nervous system of rodents .
### Output Text:
However , there is no research on <span class="chemical">GFC</span> effects in the central nervous system of rodents . 