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: The latter group was further sub - divided into 13 occult HBV ( HBsAg - negative ) and 7 overt HBV ( HBsAg - positive ) patients .
### Output Text:
The latter group was further sub - divided into 13 occult HBV ( <span class="chemical">HBsAg</span> - negative ) and 7 overt HBV ( <span class="chemical">HBsAg</span> - positive ) patients . 