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: Both DNAP and HBeAg showed a highly statistically significant correlation with the infectivity of HBsAg - positive donors .
### Output Text:
Both DNAP and <span class="chemical">HBeAg</span> showed a highly statistically significant correlation with the infectivity of <span class="chemical">HBsAg</span> - positive donors . 