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: There have been several long - term studies of patients with rheumatoid arthritis treated with azathioprine and cyclophosphamide and the incidence of most of the common cancers is not increased .
### Output Text:
There have been several long - term studies of patients with rheumatoid arthritis treated with <span class="chemical">azathioprine</span> and <span class="chemical">cyclophosphamide</span> and the incidence of most of the common cancers is not increased . 