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: Ninety - six patients were not given anticonvulsive prophylaxis ; 7 ( 7 . 5 % ) developed seizures during the 4 days of the busulfan course or within 24 h after the last dosing .
### Output Text:
Ninety - six patients were not given anticonvulsive prophylaxis ; 7 ( 7 . 5 % ) developed seizures during the 4 days of the <span class="chemical">busulfan</span> course or within 24 h after the last dosing . 