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: However , in vivo and in vitro studies have demonstrated that myometrial cells are also targets of the relaxant effects of nitric oxide ( NO ) .
### Output Text:
However , in vivo and in vitro studies have demonstrated that myometrial cells are also targets of the relaxant effects of <span class="chemical">nitric oxide</span> ( <span class="chemical">NO</span> ) . 