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: Although there have been anecdotal reports of cardiac toxicity associated with fluorouracil ( 5 - FU ) therapy , this phenomenon has not been studied in a systematic fashion .
### Output Text:
Although there have been anecdotal reports of cardiac toxicity associated with <span class="chemical">fluorouracil</span> ( <span class="chemical">5 - FU</span> ) therapy , this phenomenon has not been studied in a systematic fashion . 