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: Basal CGRP concentration was significantly higher and platelet 5 - HT content tended to be lower in subjects who experienced a migraine attack .
### Output Text:
Basal <span class="chemical">CGRP</span> concentration was significantly higher and platelet <span class="chemical">5 - HT</span> content tended to be lower in subjects who experienced a migraine attack . 