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: Administration of [ 3H ] - 5 - azadeoxycytidine during the repair synthesis induced by 1 , 2 - DMH further showed that 0 . 019 mol % of cytosine residues in DNA were substituted by the analogue , indicating that incorporation of 5 - AzC occurs during repair synthesis .
### Output Text:
Administration of <span class="chemical">[ 3H ] - 5 - azadeoxycytidine</span> during the repair synthesis induced by <span class="chemical">1 , 2 - DMH</span> further showed that 0 . 019 mol % of <span class="chemical">cytosine</span> residues in DNA were substituted by the analogue , indicating that incorporation of <span class="chemical">5 - AzC</span> occurs during repair synthesis . 