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: We report here a retrospective study of 123 children ( median age , 6 . 5 years ) receiving high - dose busulfan in combined chemotherapy before bone marrow transplantation for malignant solid tumors , brain tumors excluded .
### Output Text:
We report here a retrospective study of 123 children ( median age , 6 . 5 years ) receiving high - dose <span class="chemical">busulfan</span> in combined chemotherapy before bone marrow transplantation for malignant solid tumors , brain tumors excluded . 