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: Patients treated with alkylating agents have an increased risk of development of acute nonlymphocytic leukemia , and both alkylating agents and azathioprine are associated with the development of non - Hodgkin ' s lymphoma .
### Output Text:
Patients treated with <span class="chemical">alkylating agents</span> have an increased risk of development of acute nonlymphocytic leukemia , and both <span class="chemical">alkylating agents</span> and <span class="chemical">azathioprine</span> are associated with the development of non - Hodgkin ' s lymphoma . 