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: The present study indicates that NO - induced immediate headache is not associated with release of CGRP .
### Output Text:
The present study indicates that <span class="chemical">NO</span> - induced immediate headache is not associated with release of CGRP . 