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: IMPORTANCE OF THE FIELD : Fluoropyrimidines , in particular 5 - fluorouracil ( 5 - FU ) , have been the mainstay of treatment for several solid tumors , including colorectal , breast and head and neck cancers , for > 40 years .
### Output Text:
IMPORTANCE OF THE FIELD : <span class="chemical">Fluoropyrimidines</span> , in particular <span class="chemical">5 - fluorouracil</span> ( <span class="chemical">5 - FU</span> ) , have been the mainstay of treatment for several solid tumors , including colorectal , breast and head and neck cancers , for > 40 years . 