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: In this study , we developed a new hypoxia - responsive reporter vector using a hypoxia - responsive element of the 5 ' vascular endothelial growth factor untranslated region and generated a novel hypoxia - sensing transgenic rat .
### Output Text:
In this study , we developed a new hypoxia - responsive reporter vector using a hypoxia - responsive element of the 5 ' vascular endothelial growth factor untranslated region and generated a novel hypoxia - sensing transgenic rat . 