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: RESULTS : Five of the six patients experienced improved sexual functioning after taking yohimbine .
### Output Text:
RESULTS : Five of the six patients experienced improved sexual functioning after taking <span class="chemical">yohimbine</span> . 