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: During this period , no drug resistant mutant HBV emerged , and the serum HBV - DNA level was continuously suppressed .
### Output Text:
During this period , no drug resistant mutant HBV emerged , and the serum HBV - DNA level was continuously suppressed . 