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: Relative efficacy and toxicity of netilmicin and tobramycin in oncology patients .
### Output Text:
Relative efficacy and toxicity of <span class="chemical">netilmicin</span> and <span class="chemical">tobramycin</span> in oncology patients . 