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: Serotonin 6 receptor gene is associated with methamphetamine - induced psychosis in a Japanese population .
### Output Text:
<span class="chemical">Serotonin</span> 6 receptor gene is associated with <span class="chemical">methamphetamine</span> - induced psychosis in a Japanese population . 