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: Vascular eNOS mRNA was assessed by semi - quantitative reverse transcription - polymerase chain reaction .
### Output Text:
Vascular eNOS mRNA was assessed by semi - quantitative reverse transcription - polymerase chain reaction . 