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: Three men ( 15 % ) showed a decline in serum PSA of at least 50 % , sustained throughout treatment .
### Output Text:
Three men ( 15 % ) showed a decline in serum PSA of at least 50 % , sustained throughout treatment . 