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: Chemical analysis of gall stones retrieved at cholecystectomy from two patients , showed that they contained 71 % and 87 % cholesterol by weight .
### Output Text:
Chemical analysis of gall stones retrieved at cholecystectomy from two patients , showed that they contained 71 % and 87 % <span class="chemical">cholesterol</span> by weight . 