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: A widely used chemotherapeutic agent , 5 - fluorouracil ( 5 - FU ) , readily crosses the blood - brain barrier and so could have a direct effect on brain function .
### Output Text:
A widely used chemotherapeutic agent , <span class="chemical">5 - fluorouracil</span> ( <span class="chemical">5 - FU</span> ) , readily crosses the blood - brain barrier and so could have a direct effect on brain function . 