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: However , one drawback of such studies in genetically deficient rats is the fact that compensatory changes with upregulation of other transporters can occur .
### Output Text:
However , one drawback of such studies in genetically deficient rats is the fact that compensatory changes with upregulation of other transporters can occur . 