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 linear regression we found that no correlation exists between seizure duration , elevation of phenytoin serum levels and cerebellar volume .
### Output Text:
Using linear regression we found that no correlation exists between seizure duration , elevation of <span class="chemical">phenytoin</span> serum levels and cerebellar volume . 