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: Increased expression and activity of Caspase - 3 was also observed in STZ treated rat which specify apoptotic cell death in hippocampus and cortex .
### Output Text:
Increased expression and activity of Caspase - 3 was also observed in <span class="chemical">STZ</span> treated rat which specify apoptotic cell death in hippocampus and cortex . 