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: Ventricular RNA content was elevated 26 % after 24 h of a single injection and reached a maximal level following 8 days of therapy .
### Output Text:
Ventricular RNA content was elevated 26 % after 24 h of a single injection and reached a maximal level following 8 days of therapy . 