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: The other patients undergoing transplantation had an average ejection fraction of 17 % + / - 6 % and a VIP level of 8 . 8 + / - 3 . 9 pg / mg protein .
### Output Text:
The other patients undergoing transplantation had an average ejection fraction of 17 % + / - 6 % and a VIP level of 8 . 8 + / - 3 . 9 pg / mg protein . 