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: In the transgenic group , kanamycin increased the threshold by only 15 dB over the respective controls .
### Output Text:
In the transgenic group , <span class="chemical">kanamycin</span> increased the threshold by only 15 dB over the respective controls . 