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: The anaemia induced by 3 ' - azido - 3 ' dideoxythymidine ( AZT ) is poorly understood .
### Output Text:
The anaemia induced by <span class="chemical">3 ' - azido - 3 ' dideoxythymidine</span> ( <span class="chemical">AZT</span> ) is poorly understood . 