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 extent of inhibition of brain cholinesterase activity evoked by DCE at the dose of 400 mg / kg was 22 % in young and 19 % in aged mice .
### Output Text:
The extent of inhibition of brain cholinesterase activity evoked by <span class="chemical">DCE</span> at the dose of 400 mg / kg was 22 % in young and 19 % in aged mice . 