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: This retrospective study describes toxicity associated with a protocol of lomustine ( CCNU ) and cyclophosphamide ( CTX ) in dogs with lymphoma .
### Output Text:
This retrospective study describes toxicity associated with a protocol of <span class="chemical">lomustine</span> ( <span class="chemical">CCNU</span> ) and <span class="chemical">cyclophosphamide</span> ( <span class="chemical">CTX</span> ) in dogs with lymphoma . 