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: Quantitative analyses of labeled neurons in three regions of the dentate gyrus in the 1 and 2 week groups showed statistically significant decreases in the mean number of GAD mRNA - containing neurons in the hilus of both groups of experimental animals .
### Output Text:
Quantitative analyses of labeled neurons in three regions of the dentate gyrus in the 1 and 2 week groups showed statistically significant decreases in the mean number of GAD mRNA - containing neurons in the hilus of both groups of experimental animals . 