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: When the total busulfan dose was taken into account , there was a significant difference in terms of neurotoxicity incidence among patients under 16 mg / kg ( 1 of 57 , 1 . 7 % ) and patients under 600 mg / m2 ( 6 of 39 , 15 . 4 % ) ( P less than 0 . 02 ) .
### Output Text:
When the total <span class="chemical">busulfan</span> dose was taken into account , there was a significant difference in terms of neurotoxicity incidence among patients under 16 mg / kg ( 1 of 57 , 1 . 7 % ) and patients under 600 mg / m2 ( 6 of 39 , 15 . 4 % ) ( P less than 0 . 02 ) . 