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: It is concluded that patients on 5 - FU treatment should be under close supervision and that the treatment should be discontinued if chest pain or tachyarrhythmia is observed .
### Output Text:
It is concluded that patients on <span class="chemical">5 - FU</span> treatment should be under close supervision and that the treatment should be discontinued if chest pain or tachyarrhythmia is observed . 