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: Nature , time course and dose dependence of zidovudine - related side effects : results from the Multicenter Canadian Azidothymidine Trial .
### Output Text:
Nature , time course and dose dependence of <span class="chemical">zidovudine</span> - related side effects : results from the Multicenter Canadian <span class="chemical">Azidothymidine</span> Trial . 