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: Despite the anaemia , the number of splenic and bone marrow BFU - e in AZT treated mice increased up to five - fold over levels observed in infected untreated animals after 15 d of treatment .
### Output Text:
Despite the anaemia , the number of splenic and bone marrow BFU - e in <span class="chemical">AZT</span> treated mice increased up to five - fold over levels observed in infected untreated animals after 15 d of treatment . 