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: BACKGROUND : Incidence and risk factors for delirium during clozapine treatment require further clarification .
### Output Text:
BACKGROUND : Incidence and risk factors for delirium during <span class="chemical">clozapine</span> treatment require further clarification . 