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: In view of the known tendency of busulfan to induce cellular atypia and carcinoma in other sites , periodic urinary cytology is suggested in patients on long - term therapy .
### Output Text:
In view of the known tendency of <span class="chemical">busulfan</span> to induce cellular atypia and carcinoma in other sites , periodic urinary cytology is suggested in patients on long - term therapy . 