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 AUCCGRP recorded on the GTN day did not differ between patients and controls ( P = 0 . 36 ) .
### Output Text:
The AUCCGRP recorded on the <span class="chemical">GTN</span> day did not differ between patients and controls ( P = 0 . 36 ) . 