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: These results indicate that behavioral toxicity testing of alkylxanthines in a single mouse strain may be misleading and suggest that toxic responses of the central nervous system to this class of compounds are genetically influenced in mammals .
### Output Text:
These results indicate that behavioral toxicity testing of <span class="chemical">alkylxanthines</span> in a single mouse strain may be misleading and suggest that toxic responses of the central nervous system to this class of compounds are genetically influenced in mammals . 