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: Reduced sodium channel density , altered voltage dependence of inactivation , and increased susceptibility to seizures in mice lacking sodium channel beta 2 - subunits .
### Output Text:
Reduced <span class="chemical">sodium</span> channel density , altered voltage dependence of inactivation , and increased susceptibility to seizures in mice lacking <span class="chemical">sodium</span> channel beta 2 - subunits . 