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: DESIGN : The medical records of 206 dogs that were treated with CCNU at the Melbourne Veterinary Specialist Centre between February 2002 and December 2007 were retrospectively evaluated .
### Output Text:
DESIGN : The medical records of 206 dogs that were treated with <span class="chemical">CCNU</span> at the Melbourne Veterinary Specialist Centre between February 2002 and December 2007 were retrospectively evaluated . 