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: PGE2 synthesis was significantly higher in outer medulla , but not cortex or inner medulla , of jj ( 38 + / - 6 ng / mg prot ) than jJ rats ( 15 + / - 3 ) ( p less than 0 . 01 ) .
### Output Text:
<span class="chemical">PGE2</span> synthesis was significantly higher in outer medulla , but not cortex or inner medulla , of jj ( 38 + / - 6 ng / mg prot ) than jJ rats ( 15 + / - 3 ) ( p less than 0 . 01 ) . 