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: Serum samples were PCR amplified with HBV reverse transcriptase ( RT ) primers , followed by direct sequencing across the tyrosine - methionine - aspartate - aspartate ( YMDD ) motif of the major catalytic region in the C domain of the HBV RT enzyme .
### Output Text:
Serum samples were PCR amplified with HBV reverse transcriptase ( RT ) primers , followed by direct sequencing across the <span class="chemical">tyrosine</span> - <span class="chemical">methionine</span> - <span class="chemical">aspartate</span> - <span class="chemical">aspartate</span> ( YMDD ) motif of the major catalytic region in the C domain of the HBV RT enzyme . 