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: BACKGROUND : The 24 - hour continuous infusion of 5 - fluorouracil ( 5 - FU ) and folinic acid ( FA ) as part of several new multidrug chemotherapy regimens in advanced gastric cancer ( AGC ) has shown to be effective , with low toxicity .
### Output Text:
BACKGROUND : The 24 - hour continuous infusion of <span class="chemical">5 - fluorouracil</span> ( <span class="chemical">5 - FU</span> ) and <span class="chemical">folinic acid</span> ( <span class="chemical">FA</span> ) as part of several new multidrug chemotherapy regimens in advanced gastric cancer ( AGC ) has shown to be effective , with low toxicity . 