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: Given VIP ' s role as an anti - inflammatory mediator , we hypothesized that VIP ( - / - ) mice would exhibit enhanced inflammatory mediator expression after cystitis .
### Output Text:
Given VIP ' s role as an anti - inflammatory mediator , we hypothesized that VIP ( - / - ) mice would exhibit enhanced inflammatory mediator expression after cystitis . 