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: Its mechanism of action is not well understood although it has been proposed that lindane acts as a non - competitive antagonist at the gamma - aminobutyric acid ( GABA ) - A receptor .
### Output Text:
Its mechanism of action is not well understood although it has been proposed that <span class="chemical">lindane</span> acts as a non - competitive antagonist at the <span class="chemical">gamma - aminobutyric acid</span> ( <span class="chemical">GABA</span> ) - A receptor . 