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: Our aim was to characterize both lines ' sensitivities to various physiological effects of other ligands of the GABA ( A ) receptor .
### Output Text:
Our aim was to characterize both lines ' sensitivities to various physiological effects of other ligands of the <span class="chemical">GABA</span> ( A ) receptor . 