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 expression of VEGF receptor ( VEGFR - 2 / Flk - 1 / KDR ) was also examined by immunohistochemistry .
### Output Text:
The expression of VEGF receptor ( VEGFR - 2 / Flk - 1 / KDR ) was also examined by immunohistochemistry . 