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: Rate of YMDD motif mutants in lamivudine - untreated Iranian patients with chronic hepatitis B virus infection .
### Output Text:
Rate of YMDD motif mutants in <span class="chemical">lamivudine</span> - untreated Iranian patients with chronic hepatitis B virus infection . 