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: In vitro , gamma - HCH , pentylenetetrazol and picrotoxin were shown to inhibit 3H - TBOB binding in mouse whole brain , with IC50 values of 4 . 6 , 404 and 9 . 4 microM , respectively .
### Output Text:
In vitro , <span class="chemical">gamma - HCH</span> , <span class="chemical">pentylenetetrazol</span> and <span class="chemical">picrotoxin</span> were shown to inhibit <span class="chemical">3H - TBOB</span> binding in mouse whole brain , with IC50 values of 4 . 6 , 404 and 9 . 4 microM , respectively . 