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 use and toxicity of didanosine ( ddI ) in HIV antibody - positive individuals intolerant to zidovudine ( AZT )
### Output Text:
The use and toxicity of <span class="chemical">didanosine</span> ( <span class="chemical">ddI</span> ) in HIV antibody - positive individuals intolerant to <span class="chemical">zidovudine</span> ( <span class="chemical">AZT</span> ) 