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: Behavioural effects were tested using a context dependent conditioned emotional response test ( CER ) which showed that animals treated with 5 - FU had a significant reduction in freezing time compared to controls .
### Output Text:
Behavioural effects were tested using a context dependent conditioned emotional response test ( CER ) which showed that animals treated with <span class="chemical">5 - FU</span> had a significant reduction in freezing time compared to controls . 