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: Expression of the hypoxia - responsive transgene increased throughout the observation period , reaching 2 . 2 - fold at 2 weeks in the puromycin aminonucleoside model and 2 . 6 - fold at 4 weeks in the remnant kidney model , whereas that of vascular endothelial growth factor showed a mild decrease , reflecting distinct behaviors of the two genes .
### Output Text:
Expression of the hypoxia - responsive transgene increased throughout the observation period , reaching 2 . 2 - fold at 2 weeks in the <span class="chemical">puromycin aminonucleoside</span> model and 2 . 6 - fold at 4 weeks in the remnant kidney model , whereas that of vascular endothelial growth factor showed a mild decrease , reflecting distinct behaviors of the two genes . 