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: 5 - FU chemotherapy caused a significant reduction in the number of proliferating cells in the sub granular zone of the dentate gyrus compared to controls .
### Output Text:
<span class="chemical">5 - FU</span> chemotherapy caused a significant reduction in the number of proliferating cells in the sub granular zone of the dentate gyrus compared to controls . 