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: Sodium channel beta - subunits modulate channel gating , assembly , and cell surface expression in heterologous cell systems .
### Output Text:
<span class="chemical">Sodium</span> channel beta - subunits modulate channel gating , assembly , and cell surface expression in heterologous cell systems . 