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: It has been proposed that nitric oxide ( NO ) induced headache in primary headaches may be associated with release of calcitonin gene - related peptide ( CGRP ) .
### Output Text:
It has been proposed that <span class="chemical">nitric oxide</span> ( <span class="chemical">NO</span> ) induced headache in primary headaches may be associated with release of calcitonin gene - related peptide ( CGRP ) . 