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: Studies were performed to examine the mechanisms for the protective effects of free radical scavengers on gentamicin ( GM ) - mediated nephropathy .
### Output Text:
Studies were performed to examine the mechanisms for the protective effects of free radical scavengers on <span class="chemical">gentamicin</span> ( <span class="chemical">GM</span> ) - mediated nephropathy . 