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: CCNU ( lomustine ) toxicity in dogs : a retrospective study ( 2002 - 07 ) .
### Output Text:
<span class="chemical">CCNU</span> ( <span class="chemical">lomustine</span> ) toxicity in dogs : a retrospective study ( 2002 - 07 ) . 