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: This is evident by Nrf2 ( - / - ) mice having higher ROS production and suffering from greater oxidative DNA damage and renal injury compared with Nrf2 ( + / + ) mice .
### Output Text:
This is evident by Nrf2 ( - / - ) mice having higher ROS production and suffering from greater oxidative DNA damage and renal injury compared with Nrf2 ( + / + ) mice . 