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: Instead , oral administration of S - 1 ( a derivative of 5 - FU ) , at 200 mg / day twice a week , was instituted , because S - 1 has a strong inhibitory effect on dihydropyrimidine dehydrogenase , which catalyzes the degradative of 5 - FU into FBAL .
### Output Text:
Instead , oral administration of S - 1 ( a derivative of <span class="chemical">5 - FU</span> ) , at 200 mg / day twice a week , was instituted , because S - 1 has a strong inhibitory effect on <span class="chemical">dihydropyrimidine</span> dehydrogenase , which catalyzes the degradative of <span class="chemical">5 - FU</span> into <span class="chemical">FBAL</span> . 