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: CBA / Ca male mice started on AZT 0 . 75 mg / ml H2O at 84 days of age and kept on it for 687 days when dosage reduced to 0 . 5 mg / ml H2O for a group , another group removed from AZT to see recovery , and third group remained on 0 . 75 mg .
### Output Text:
CBA / Ca male mice started on <span class="chemical">AZT</span> 0 . 75 mg / ml H2O at 84 days of age and kept on it for 687 days when dosage reduced to 0 . 5 mg / ml H2O for a group , another group removed from <span class="chemical">AZT</span> to see recovery , and third group remained on 0 . 75 mg . 