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 aim of this study was to examine the impact of anemia prevention by recombinant human erythropoietin ( rHuEPO ) treatment on the cytotoxicity of cyclophosphamide in solid experimental tumors .
### Output Text:
The aim of this study was to examine the impact of anemia prevention by recombinant human erythropoietin ( rHuEPO ) treatment on the cytotoxicity of <span class="chemical">cyclophosphamide</span> in solid experimental tumors . 