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: Furthermore , mRNA levels of the proinflammatory cytokine interleukin 6 were increased in the cardiac tissue .
### Output Text:
Furthermore , mRNA levels of the proinflammatory cytokine interleukin 6 were increased in the cardiac tissue . 