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: Estrogen prevents cholesteryl ester accumulation in macrophages induced by the HIV protease inhibitor ritonavir .
### Output Text:
Estrogen prevents <span class="chemical">cholesteryl ester</span> accumulation in macrophages induced by the HIV protease inhibitor <span class="chemical">ritonavir</span> . 