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: The maximum tolerated dose of cyclophosphamide was defined as 900 mg / m ( 2 ) .
### Output Text:
The maximum tolerated dose of <span class="chemical">cyclophosphamide</span> was defined as 900 mg / m ( 2 ) . 