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: There was no difference between the area under the CGRP curve ( AUCCGRP ) on GTN vs . placebo day in either patients ( P = 0 . 65 ) or controls ( P = 0 . 48 ) .
### Output Text:
There was no difference between the area under the CGRP curve ( AUCCGRP ) on <span class="chemical">GTN</span> vs . placebo day in either patients ( P = 0 . 65 ) or controls ( P = 0 . 48 ) . 