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 , plasma CGRP concentrations failed to change during immediate headache and in the subjects with no migraine attack .
### Output Text:
However , plasma <span class="chemical">CGRP</span> concentrations failed to change during immediate headache and in the subjects with no migraine attack . 