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: CB 3717 , N10 - propargyl - 5 , 8 - dideazafolic acid , is a tight - binding inhibitor of thymidylate synthase ( TS ) whose cytotoxicity is mediated solely through the inhibition of this enzyme .
### Output Text:
<span class="chemical">CB 3717</span> , <span class="chemical">N10 - propargyl - 5 , 8 - dideazafolic acid</span> , is a tight - binding inhibitor of thymidylate synthase ( TS ) whose cytotoxicity is mediated solely through the inhibition of this enzyme . 