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 , MK - 801 injection produced a significant increase of tremor in reserpine - treated mice .
### Output Text:
However , <span class="chemical">MK - 801</span> injection produced a significant increase of tremor in <span class="chemical">reserpine</span> - treated mice . 