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 conclusion , the fact that plasma CGRP concentration correlates with the timing and severity of a migraine headache suggests a direct relationship between CGRP and migraine .
### Output Text:
In conclusion , the fact that plasma <span class="chemical">CGRP</span> concentration correlates with the timing and severity of a migraine headache suggests a direct relationship between <span class="chemical">CGRP</span> and migraine . 