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: Recent reports indicate that single agent therapy with vinorelbine ( VNB ) or gemcitabine ( GEM ) may obtain a response rate of 20 - 30 % in elderly patients , with acceptable toxicity and improvement in symptoms and quality of life .
### Output Text:
Recent reports indicate that single agent therapy with <span class="chemical">vinorelbine</span> ( <span class="chemical">VNB</span> ) or <span class="chemical">gemcitabine</span> ( <span class="chemical">GEM</span> ) may obtain a response rate of 20 - 30 % in elderly patients , with acceptable toxicity and improvement in symptoms and quality of life . 