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: In recent years , increasing evidence has suggested that the messenger molecule nitric oxide ( NO ) is involved in pain mechanisms of migraine without aura .
### Output Text:
In recent years , increasing evidence has suggested that the messenger molecule <span class="chemical">nitric oxide</span> ( <span class="chemical">NO</span> ) is involved in pain mechanisms of migraine without aura . 