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: A relative gamma - aminobutyric acid - ergic deficiency might occur because diazepam , a gamma - aminobutyric acid - mimetic agent , was strikingly effective .
### Output Text:
A relative <span class="chemical">gamma - aminobutyric acid</span> - ergic deficiency might occur because <span class="chemical">diazepam</span> , a <span class="chemical">gamma - aminobutyric acid</span> - mimetic agent , was strikingly effective . 