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: Sensitivity of erythroid progenitor colonies to erythropoietin in azidothymidine treated immunodeficient mice .
### Output Text:
Sensitivity of erythroid progenitor colonies to erythropoietin in <span class="chemical">azidothymidine</span> treated immunodeficient mice . 