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: Median follow - up from the start of GEM - P was 4 . 5 years .
### Output Text:
Median follow - up from the start of <span class="chemical">GEM</span> - P was 4 . 5 years . 