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: Using mice derived from a classical F2 and backcross genetic design , a relationship between nicotine - induced seizures and alpha - bungarotoxin nicotinic receptor concentration was found .
### Output Text:
Using mice derived from a classical F2 and backcross genetic design , a relationship between <span class="chemical">nicotine</span> - induced seizures and alpha - bungarotoxin nicotinic receptor concentration was found . 