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: PURPOSE : We gained knowledge of the etiology , treatment and prevention of cyclophosphamide associated urothelial cancer .
### Output Text:
PURPOSE : We gained knowledge of the etiology , treatment and prevention of <span class="chemical">cyclophosphamide</span> associated urothelial cancer . 