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: In the absence of the carcinogen , 5 - AzC given after a two thirds partial hepatectomy , when its incorporation should be maximum , failed to induce any gamma - GT positive foci .
### Output Text:
In the absence of the carcinogen , <span class="chemical">5 - AzC</span> given after a two thirds partial hepatectomy , when its incorporation should be maximum , failed to induce any gamma - GT positive foci . 