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: Dose - dependent neurotoxicity of high - dose busulfan in children : a clinical and pharmacological study .
### Output Text:
Dose - dependent neurotoxicity of high - dose <span class="chemical">busulfan</span> in children : a clinical and pharmacological study . 