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: GEM - P is a salvage chemotherapy with relatively high response rates , leading to successful transplantation in appropriate patients , in the treatment of relapsed or refractory HL .
### Output Text:
<span class="chemical">GEM</span> - P is a salvage chemotherapy with relatively high response rates , leading to successful transplantation in appropriate patients , in the treatment of relapsed or refractory HL . 