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: To the best of our knowledge , this constitutes the first report of HBV lamivudine - resistant strains in therapy - na ve HBV - HIV co - infected patients .
### Output Text:
To the best of our knowledge , this constitutes the first report of HBV <span class="chemical">lamivudine</span> - resistant strains in therapy - <span class="chemical">na</span> ve HBV - HIV co - infected patients . 