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: BACKGROUND : Combined hepatitis B immune globulin ( HBIg ) and lamivudine in prophylaxis of the recurrence of hepatitis B after liver transplantation has significantly improved the survival of HBsAg positive patients .
### Output Text:
BACKGROUND : Combined hepatitis B immune globulin ( HBIg ) and <span class="chemical">lamivudine</span> in prophylaxis of the recurrence of hepatitis B after liver transplantation has significantly improved the survival of <span class="chemical">HBsAg</span> positive patients . 