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: Colony formation by splenic and bone marrow BFUe was stimulated at lower concentrations of EPO in mice receiving AZT for 15 d than for infected , untreated mice .
### Output Text:
Colony formation by splenic and bone marrow BFUe was stimulated at lower concentrations of EPO in mice receiving <span class="chemical">AZT</span> for 15 d than for infected , untreated mice . 