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: CONCLUSION : These data indicate that VNB is an active agent in metastatic esophageal squamous cell carcinoma .
### Output Text:
CONCLUSION : These data indicate that <span class="chemical">VNB</span> is an active agent in metastatic esophageal squamous cell carcinoma . 