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: CNS toxic effects of the antineoplastic agent ifosfamide ( IFX ) are frequent and include a variety of neurological symptoms that can limit drug use .
### Output Text:
CNS toxic effects of the antineoplastic agent <span class="chemical">ifosfamide</span> ( <span class="chemical">IFX</span> ) are frequent and include a variety of neurological symptoms that can limit drug use . 