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: Our observations show that beta2 - subunits play an important role in the regulation of sodium channel density and function in neurons in vivo and are required for normal action potential generation and control of excitability .
### Output Text:
Our observations show that beta2 - subunits play an important role in the regulation of <span class="chemical">sodium</span> channel density and function in neurons in vivo and are required for normal action potential generation and control of excitability . 