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: Paclitaxel and 5 - fluorouracil have additive cytotoxicity in MCF - 7 cell lines .
### Output Text:
<span class="chemical">Paclitaxel</span> and <span class="chemical">5 - fluorouracil</span> have additive cytotoxicity in MCF - 7 cell lines . 