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: With a median follow - up of 26 months , the 2 - year overall survival rate was 47 % .
### Output Text:
With a median follow - up of 26 months , the 2 - year overall survival rate was 47 % . 