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: Nephrotoxicity occurred in a similar proportion in patients treated with netilmicin and tobramycin ( 17 % vs 11 % ) .
### Output Text:
Nephrotoxicity occurred in a similar proportion in patients treated with <span class="chemical">netilmicin</span> and <span class="chemical">tobramycin</span> ( 17 % vs 11 % ) . 