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: We prospectively performed continuous ambulatory ECG monitoring on 25 patients undergoing 5 - FU infusion for treatment of solid tumors in order to assess the incidence of ischemic ST changes .
### Output Text:
We prospectively performed continuous ambulatory ECG monitoring on 25 patients undergoing <span class="chemical">5 - FU</span> infusion for treatment of solid tumors in order to assess the incidence of ischemic ST changes . 