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: There is a < 2 dB difference in mean threshold for patients treated with DFMO plus sulindac compared with those treated with placebo .
### Output Text:
There is a < 2 dB difference in mean threshold for patients treated with <span class="chemical">DFMO</span> plus <span class="chemical">sulindac</span> compared with those treated with placebo . 