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: Nested within the cohort , a matched case - control study was performed to estimate the association between cyclophosphamide and bladder cancer using odds ratios ( ORs ) as relative risk .
### Output Text:
Nested within the cohort , a matched case - control study was performed to estimate the association between <span class="chemical">cyclophosphamide</span> and bladder cancer using odds ratios ( ORs ) as relative risk . 