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: CONCLUSIONS : The combination of GEM and VNB is moderately active and well tolerated except in patients age > / = 75 years .
### Output Text:
CONCLUSIONS : The combination of <span class="chemical">GEM</span> and <span class="chemical">VNB</span> is moderately active and well tolerated except in patients age > / = 75 years . 