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: This hypothesis stems from findings that PREGS is a potent positive modulator of N - methyl - d - aspartate receptors ( NMDARs ) and a negative modulator of gamma - aminobutyric acid ( A ) receptors ( GABA ( A ) Rs ) .
### Output Text:
This hypothesis stems from findings that <span class="chemical">PREGS</span> is a potent positive modulator of <span class="chemical">N - methyl - d - aspartate</span> receptors ( NMDARs ) and a negative modulator of <span class="chemical">gamma - aminobutyric acid</span> ( A ) receptors ( <span class="chemical">GABA</span> ( A ) Rs ) . 