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: alpha3 + / - mice were partially resistant to nicotine - induced seizures when compared to wild - type littermates .
### Output Text:
alpha3 + / - mice were partially resistant to <span class="chemical">nicotine</span> - induced seizures when compared to wild - type littermates . 