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: BACKGROUND : Preclinical and clinical studies suggest that yohimbine facilitates sexual behavior and may be helpful in the treatment of male impotence .
### Output Text:
BACKGROUND : Preclinical and clinical studies suggest that <span class="chemical">yohimbine</span> facilitates sexual behavior and may be helpful in the treatment of male impotence . 