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 Alzheimer ' s disease groups , rats were injected with STZ - icv bilaterally ( 3 mg / kg ) in first day and 3 days later , a similar STZ - icv application was repeated .
### Output Text:
In Alzheimer ' s disease groups , rats were injected with <span class="chemical">STZ</span> - icv bilaterally ( 3 mg / kg ) in first day and 3 days later , a similar <span class="chemical">STZ</span> - icv application was repeated . 