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: Lamivudine for the prevention of hepatitis B virus reactivation in hepatitis - B surface antigen ( HBSAG ) seropositive cancer patients undergoing cytotoxic chemotherapy .
### Output Text:
<span class="chemical">Lamivudine</span> for the prevention of hepatitis B virus reactivation in <span class="chemical">hepatitis - B surface antigen</span> ( <span class="chemical">HBSAG</span> ) seropositive cancer patients undergoing cytotoxic chemotherapy . 