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: Measurements of [ ( 3 ) H ] saxitoxin ( STX ) binding showed a significant reduction in the level of plasma membrane sodium channels in beta2 ( - / - ) neurons .
### Output Text:
Measurements of [ ( 3 ) H ] <span class="chemical">saxitoxin</span> ( <span class="chemical">STX</span> ) binding showed a significant reduction in the level of plasma membrane <span class="chemical">sodium</span> channels in beta2 ( - / - ) neurons . 