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: RESEARCH DESIGN AND METHODS : We explore the protective role of Nrf2 against diabetic nephropathy using human kidney biopsy tissues from diabetic nephropathy patients , a streptozotocin - induced diabetic nephropathy model in Nrf2 ( - / - ) mice , and cultured human mesangial cells .
### Output Text:
RESEARCH DESIGN AND METHODS : We explore the protective role of Nrf2 against diabetic nephropathy using human kidney biopsy tissues from diabetic nephropathy patients , a <span class="chemical">streptozotocin</span> - induced diabetic nephropathy model in Nrf2 ( - / - ) mice , and cultured human mesangial cells . 