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 from patients were tested by polymerase chain reaction - restriction fragment length polymorphism ( PCR - RFLP ) for detection of YMDD motif mutants .
### Output Text:
Serum samples from patients were tested by polymerase chain reaction - restriction fragment length polymorphism ( PCR - RFLP ) for detection of YMDD motif mutants . 