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: Urinary protein excretion in the PAN group increased significantly , peaking at 8 days , then decreased at 4 weeks , but rose again significantly at 14 and 20 weeks .
### Output Text:
Urinary protein excretion in the <span class="chemical">PAN</span> group increased significantly , peaking at 8 days , then decreased at 4 weeks , but rose again significantly at 14 and 20 weeks . 