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: A significant up - regulation of alphaENaC and betaENaC mRNA abundance on days 1 and 2 preceded sodium retention on days 2 and 3 .
### Output Text:
A significant up - regulation of alphaENaC and betaENaC mRNA abundance on days 1 and 2 preceded <span class="chemical">sodium</span> retention on days 2 and 3 . 