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: The pharmacological challenge data suggest that tolerance may occur to seizure activity induced by PTZ and PTX 24 h after gamma - HCH , since the response to only these two seizure - inducing agents is decreased .
### Output Text:
The pharmacological challenge data suggest that tolerance may occur to seizure activity induced by <span class="chemical">PTZ</span> and <span class="chemical">PTX</span> 24 h after <span class="chemical">gamma - HCH</span> , since the response to only these two seizure - inducing agents is decreased . 