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: CONCLUSIONS : NGF and PGs have important roles in the development of OAB symptoms in female patients .
### Output Text:
CONCLUSIONS : NGF and <span class="chemical">PGs</span> have important roles in the development of OAB symptoms in female patients . 