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: Additionally , ritonavir treatment selectively increased the relative levels of PPARgamma mRNA , a transcription factor responsible for the regulation of CD36 mRNA expression .
### Output Text:
Additionally , <span class="chemical">ritonavir</span> treatment selectively increased the relative levels of PPARgamma mRNA , a transcription factor responsible for the regulation of CD36 mRNA expression . 