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: STZ treatment showed decrease expression of post synaptic markers CaMKIIa and PSD - 95 , while , expression of pre synaptic markers ( synaptophysin and SNAP - 25 ) remains unaltered indicating selective post synaptic neurotoxicity .
### Output Text:
<span class="chemical">STZ</span> treatment showed decrease expression of post synaptic markers CaMKIIa and PSD - 95 , while , expression of pre synaptic markers ( synaptophysin and SNAP - 25 ) remains unaltered indicating selective post synaptic neurotoxicity . 